diff --git a/polygerrit-ui/app/api/diff.ts b/polygerrit-ui/app/api/diff.ts
index 6788aa3..4322c64 100644
--- a/polygerrit-ui/app/api/diff.ts
+++ b/polygerrit-ui/app/api/diff.ts
@@ -38,10 +38,16 @@
  * If the weblinks-only parameter is specified, only the web_links field is set.
  */
 export declare interface DiffInfo {
-  /** Meta information about the file on side A as a DiffFileMetaInfo entity. */
-  meta_a: DiffFileMetaInfo;
-  /** Meta information about the file on side B as a DiffFileMetaInfo entity. */
-  meta_b: DiffFileMetaInfo;
+  /**
+   * Meta information about the file on side A as a DiffFileMetaInfo entity.
+   * Not set when change_type is ADDED.
+   */
+  meta_a?: DiffFileMetaInfo;
+  /**
+   * Meta information about the file on side B as a DiffFileMetaInfo entity.
+   * Not set when change_type is DELETED.
+   */
+  meta_b?: DiffFileMetaInfo;
   /** The type of change (ADDED, MODIFIED, DELETED, RENAMED COPIED, REWRITE). */
   change_type: ChangeType;
   /** Intraline status (OK, ERROR, TIMEOUT). */
@@ -167,7 +173,7 @@
    * Indicates the range (line numbers) on the other side of the comparison
    * where the code related to the current chunk came from/went to.
    */
-  range: {
+  range?: {
     start: number;
     end: number;
   };
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host_test.js b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host_test.js
index dda8490..88aae5c 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host_test.js
+++ b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host_test.js
@@ -313,7 +313,7 @@
       // Recognizes that it should be an image diff.
       assert.isTrue(element.isImageDiff);
       assert.instanceOf(
-          element.$.diff.$.diffBuilder._builder, GrDiffBuilderImage);
+          element.$.diff.diffBuilder.builder, GrDiffBuilderImage);
 
       // Left image rendered with the parent commit's version of the file.
       const leftImage =
@@ -381,7 +381,7 @@
       // Recognizes that it should be an image diff.
       assert.isTrue(element.isImageDiff);
       assert.instanceOf(
-          element.$.diff.$.diffBuilder._builder, GrDiffBuilderImage);
+          element.$.diff.diffBuilder.builder, GrDiffBuilderImage);
 
       // Left image rendered with the parent commit's version of the file.
       const leftImage =
@@ -445,7 +445,7 @@
         // Recognizes that it should be an image diff.
         assert.isTrue(element.isImageDiff);
         assert.instanceOf(
-            element.$.diff.$.diffBuilder._builder, GrDiffBuilderImage);
+            element.$.diff.diffBuilder.builder, GrDiffBuilderImage);
 
         const leftImage =
             element.$.diff.$.diffTable.querySelector('td.left img');
@@ -493,7 +493,7 @@
         // Recognizes that it should be an image diff.
         assert.isTrue(element.isImageDiff);
         assert.instanceOf(
-            element.$.diff.$.diffBuilder._builder, GrDiffBuilderImage);
+            element.$.diff.diffBuilder.builder, GrDiffBuilderImage);
 
         const leftImage =
             element.$.diff.$.diffTable.querySelector('td.left img');
@@ -543,7 +543,7 @@
         // Recognizes that it should be an image diff.
         assert.isTrue(element.isImageDiff);
         assert.instanceOf(
-            element.$.diff.$.diffBuilder._builder, GrDiffBuilderImage);
+            element.$.diff.diffBuilder.builder, GrDiffBuilderImage);
         const leftImage =
             element.$.diff.$.diffTable.querySelector('td.left img');
         assert.isNotOk(leftImage);
@@ -623,7 +623,7 @@
 
     test('clearBlame', () => {
       element._blame = [];
-      const setBlameSpy = sinon.spy(element.$.diff.$.diffBuilder, 'setBlame');
+      const setBlameSpy = sinon.spy(element.$.diff.diffBuilder, 'setBlame');
       element.clearBlame();
       assert.isNull(element._blame);
       assert.isTrue(setBlameSpy.calledWithExactly(null));
diff --git a/polygerrit-ui/app/embed/diff/gr-context-controls/gr-context-controls.ts b/polygerrit-ui/app/embed/diff/gr-context-controls/gr-context-controls.ts
index a5effaf..47f5f81 100644
--- a/polygerrit-ui/app/embed/diff/gr-context-controls/gr-context-controls.ts
+++ b/polygerrit-ui/app/embed/diff/gr-context-controls/gr-context-controls.ts
@@ -462,7 +462,7 @@
     numLines: number,
     referenceLine: number
   ) {
-    assertIsDefined(this.diff, 'diff');
+    if (!this.diff?.meta_b) return;
     const syntaxTree = this.diff.meta_b.syntax_tree;
     const outlineSyntaxPath = findBlockTreePathForLine(
       referenceLine,
diff --git a/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-element.ts b/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-element.ts
index 269b56d..d200c75 100644
--- a/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-element.ts
+++ b/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-element.ts
@@ -1,24 +1,11 @@
 /**
  * @license
- * Copyright (C) 2016 The Android Open Source Project
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- * http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
+ * Copyright 2016 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
  */
 import '../gr-diff-processor/gr-diff-processor';
 import '../../../elements/shared/gr-hovercard/gr-hovercard';
 import './gr-diff-builder-side-by-side';
-import {PolymerElement} from '@polymer/polymer/polymer-element';
-import {htmlTemplate} from './gr-diff-builder-element_html';
 import {GrAnnotation} from '../gr-diff-highlight/gr-annotation';
 import {DiffBuilder, DiffContextExpandedEventDetail} from './gr-diff-builder';
 import {GrDiffBuilderSideBySide} from './gr-diff-builder-side-by-side';
@@ -26,7 +13,6 @@
 import {GrDiffBuilderUnified} from './gr-diff-builder-unified';
 import {GrDiffBuilderBinary} from './gr-diff-builder-binary';
 import {CancelablePromise, makeCancelable} from '../../../scripts/util';
-import {customElement, property, observe} from '@polymer/decorators';
 import {BlameInfo, ImageInfo} from '../../../types/common';
 import {DiffInfo, DiffPreferencesInfo} from '../../../types/diff';
 import {CoverageRange, DiffLayer} from '../../../types/types';
@@ -53,18 +39,35 @@
   hideInContextControl,
 } from '../gr-diff/gr-diff-group';
 import {getLineNumber, getSideByLineEl} from '../gr-diff/gr-diff-utils';
-import {fireAlert, fireEvent, fire} from '../../../utils/event-util';
-import {afterNextRender} from '@polymer/polymer/lib/utils/render-status';
+import {
+  fireAlert,
+  fire,
+  HTMLElementEventDetailType,
+} from '../../../utils/event-util';
+import {assertIsDefined} from '../../../utils/common-util';
+import {afterNextRender} from '../../../utils/dom-util';
 
 const TRAILING_WHITESPACE_PATTERN = /\s+$/;
-
-// https://gerrit.googlesource.com/gerrit/+/234616a8627334686769f1de989d286039f4d6a5/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.js#740
 const COMMIT_MSG_PATH = '/COMMIT_MSG';
 const COMMIT_MSG_LINE_LENGTH = 72;
 
 declare global {
   interface HTMLElementEventMap {
+    /**
+     * Fired when the diff begins rendering - both for full renders and for
+     * partial rerenders.
+     */
+    'render-start': CustomEvent<{}>;
+    /**
+     * Fired whenever a new chunk of lines has been rendered synchronously - this
+     * only happens for full renders.
+     */
     'render-progress': CustomEvent<RenderProgressEventDetail>;
+    /**
+     * Fired when the diff finishes rendering text content - both for full
+     * renders and for partial rerenders.
+     */
+    'render-content': CustomEvent<{}>;
   }
 }
 
@@ -97,112 +100,59 @@
   }
 }
 
-@customElement('gr-diff-builder')
-export class GrDiffBuilderElement
-  extends PolymerElement
-  implements GroupConsumer
-{
-  static get template() {
-    return htmlTemplate;
-  }
-
-  /**
-   * Fired when the diff begins rendering - both for full renders and for
-   * partial rerenders.
-   *
-   * @event render-start
-   */
-
-  /**
-   * Fired whenever a new chunk of lines has been rendered synchronously - this
-   * only happens for full renders.
-   *
-   * @event render-progress
-   */
-
-  /**
-   * Fired when the diff finishes rendering text content - both for full
-   * renders and for partial rerenders.
-   *
-   * @event render-content
-   */
-
-  @property({type: Object})
+// TODO: Rename the class and the file and remove "element". This is not an
+// element anymore.
+export class GrDiffBuilderElement implements GroupConsumer {
   diff?: DiffInfo;
 
-  @property({type: String})
+  diffElement?: HTMLTableElement;
+
   viewMode?: string;
 
-  @property({type: Boolean})
   isImageDiff?: boolean;
 
-  @property({type: Object})
   baseImage: ImageInfo | null = null;
 
-  @property({type: Object})
   revisionImage: ImageInfo | null = null;
 
-  @property({type: Number})
-  parentIndex?: number;
-
-  @property({type: String})
   path?: string;
 
-  @property({type: Object})
   prefs: DiffPreferencesInfo = createDefaultDiffPrefs();
 
-  @property({type: Object})
   renderPrefs?: RenderPreferences;
 
-  @property({type: Object})
-  _builder?: DiffBuilder;
-
-  /**
-   * The gr-diff-processor adds (and only adds!) to this array. It does so by
-   * using `this.push()` and Polymer's two-way data binding.
-   * Below (@observe('_groups.splices')) we are observing the groups that the
-   * processor adds, and pass them on to the builder for rendering. Henceforth
-   * the builder groups are the source of truth, because when
-   * expanding/collapsing groups only the builder is updated. This field and the
-   * corresponsing one in the processor are not updated.
-   */
-  @property({type: Array})
-  _groups: GrDiffGroup[] = [];
+  useNewImageDiffUi = false;
 
   /**
    * Layers passed in from the outside.
+   *
+   * See `layersInternal` for where these layers will end up together with the
+   * internal layers.
    */
-  @property({type: Array})
   layers: DiffLayer[] = [];
 
+  // visible for testing
+  builder?: DiffBuilder;
+
   /**
-   * All layers, both from the outside and the default ones.
+   * All layers, both from the outside and the default ones. See `layers` for
+   * the property that can be set from the outside.
    */
-  @property({type: Array})
-  _layers: DiffLayer[] = [];
+  // visible for testing
+  layersInternal: DiffLayer[] = [];
 
-  @property({type: Boolean})
-  _showTabs?: boolean;
+  // visible for testing
+  showTabs?: boolean;
 
-  @property({type: Boolean})
-  _showTrailingWhitespace?: boolean;
-
-  @property({type: Array})
-  commentRanges: CommentRangeLayer[] = [];
-
-  @property({type: Array, observer: 'coverageObserver'})
-  coverageRanges: CoverageRange[] = [];
-
-  @property({type: Boolean})
-  useNewImageDiffUi = false;
+  // visible for testing
+  showTrailingWhitespace?: boolean;
 
   /**
    * The promise last returned from `render()` while the asynchronous
    * rendering is running - `null` otherwise. Provides a `cancel()`
    * method that rejects it with `{isCancelled: true}`.
    */
-  @property({type: Object})
-  _cancelableRenderPromise: CancelablePromise<unknown> | null = null;
+  private cancelableRenderPromise: CancelablePromise<unknown> | null = null;
 
   private coverageLayerLeft = new GrCoverageLayer(Side.LEFT);
 
@@ -210,51 +160,20 @@
 
   private rangeLayer = new GrRangedCommentLayer();
 
-  private processor = new GrDiffProcessor();
+  // visible for testing
+  processor = new GrDiffProcessor();
 
   constructor() {
-    super();
-    afterNextRender(this, () => {
-      this.addEventListener(
-        'diff-context-expanded',
-        (e: CustomEvent<DiffContextExpandedEventDetail>) => {
-          // Don't stop propagation. The host may listen for reporting or
-          // resizing.
-          this.replaceGroup(e.detail.contextGroup, e.detail.groups);
-        }
-      );
-    });
     this.processor.consumer = this;
   }
 
-  override disconnectedCallback() {
-    this.processor.cancel();
-    if (this._builder) {
-      this._builder.clear();
-    }
-    super.disconnectedCallback();
+  updateCommentRanges(ranges: CommentRangeLayer[]) {
+    this.rangeLayer.updateRanges(ranges);
   }
 
-  get diffElement(): HTMLTableElement {
-    // Not searching in shadowRoot, because the diff table is slotted!
-    return this.querySelector('#diffTable') as HTMLTableElement;
-  }
-
-  @observe('commentRanges.*')
-  rangeObserver() {
-    this.rangeLayer.updateRanges(this.commentRanges);
-  }
-
-  coverageObserver(coverageRanges: CoverageRange[]) {
-    const leftRanges = coverageRanges.filter(
-      range => range && range.side === Side.LEFT
-    );
-    this.coverageLayerLeft.setRanges(leftRanges);
-
-    const rightRanges = coverageRanges.filter(
-      range => range && range.side === Side.RIGHT
-    );
-    this.coverageLayerRight.setRanges(rightRanges);
+  updateCoverageRanges(rs: CoverageRange[]) {
+    this.coverageLayerLeft.setRanges(rs.filter(r => r?.side === Side.LEFT));
+    this.coverageLayerRight.setRanges(rs.filter(r => r?.side === Side.RIGHT));
   }
 
   render(keyLocations: KeyLocations): void {
@@ -262,42 +181,44 @@
     // installed, and |render| satisfies the requirement, however,
     // |attached| doesn't because in the diff view page, the element is
     // attached before plugins are installed.
-    this._setupAnnotationLayers();
+    this.setupAnnotationLayers();
 
-    this._showTabs = this.prefs.show_tabs;
-    this._showTrailingWhitespace = this.prefs.show_whitespace_errors;
+    this.showTabs = this.prefs.show_tabs;
+    this.showTrailingWhitespace = this.prefs.show_whitespace_errors;
 
     // Stop the processor if it's running.
     this.cancel();
 
-    if (this._builder) {
-      this._builder.clear();
-    }
-    if (!this.diff) {
-      throw Error('Cannot render a diff without DiffInfo.');
-    }
-    this._builder = this._getDiffBuilder();
+    this.builder?.clear();
+    assertIsDefined(this.diff, 'diff');
+    assertIsDefined(this.diffElement, 'diff table');
+    this.builder = this.getDiffBuilder();
 
     this.processor.context = this.prefs.context;
     this.processor.keyLocations = keyLocations;
 
-    this._clearDiffContent();
-    this._builder.addColumns(
+    this.diffElement.addEventListener(
+      'diff-context-expanded',
+      this.onDiffContextExpanded
+    );
+
+    this.clearDiffContent();
+    this.builder.addColumns(
       this.diffElement,
       getLineNumberCellWidth(this.prefs)
     );
 
     const isBinary = !!(this.isImageDiff || this.diff.binary);
 
-    fireEvent(this, 'render-start');
-    this._cancelableRenderPromise = makeCancelable(
+    this.fireDiffEvent('render-start', {});
+    this.cancelableRenderPromise = makeCancelable(
       this.processor
         .process(this.diff.content, isBinary)
         .then(() => {
           if (this.isImageDiff) {
-            (this._builder as GrDiffBuilderImage).renderDiff();
+            (this.builder as GrDiffBuilderImage).renderDiff();
           }
-          afterNextRender(this, () => fireEvent(this, 'render-content'));
+          afterNextRender(() => this.fireDiffEvent('render-content', {}));
         })
         // Mocha testing does not like uncaught rejections, so we catch
         // the cancels which are expected and should not throw errors in
@@ -307,17 +228,39 @@
           return;
         })
         .finally(() => {
-          this._cancelableRenderPromise = null;
+          this.cancelableRenderPromise = null;
         })
     );
   }
 
-  _setupAnnotationLayers() {
+  private onDiffContextExpanded = (
+    e: CustomEvent<DiffContextExpandedEventDetail>
+  ) => {
+    // Don't stop propagation. The host may listen for reporting or
+    // resizing.
+    this.replaceGroup(e.detail.contextGroup, e.detail.groups);
+  };
+
+  private fireDiffEvent<K extends keyof HTMLElementEventMap>(
+    type: K,
+    detail: HTMLElementEventDetailType<K>
+  ) {
+    assertIsDefined(this.diffElement, 'diff table');
+    fire(this.diffElement, type, detail);
+  }
+
+  private fireDiffEventRenderProgress(detail: RenderProgressEventDetail) {
+    assertIsDefined(this.diffElement, 'diff table');
+    fire(this.diffElement, 'render-progress', detail);
+  }
+
+  // visible for testing
+  setupAnnotationLayers() {
     const layers: DiffLayer[] = [
-      this._createTrailingWhitespaceLayer(),
-      this._createIntralineLayer(),
-      this._createTabIndicatorLayer(),
-      this._createSpecialCharacterIndicatorLayer(),
+      this.createTrailingWhitespaceLayer(),
+      this.createIntralineLayer(),
+      this.createTabIndicatorLayer(),
+      this.createSpecialCharacterIndicatorLayer(),
       this.rangeLayer,
       this.coverageLayerLeft,
       this.coverageLayerRight,
@@ -326,15 +269,15 @@
     if (this.layers) {
       layers.push(...this.layers);
     }
-    this._layers = layers;
+    this.layersInternal = layers;
   }
 
   getContentTdByLine(lineNumber: LineNumber, side?: Side, root?: Element) {
-    if (!this._builder) return null;
-    return this._builder.getContentTdByLine(lineNumber, side, root);
+    if (!this.builder) return null;
+    return this.builder.getContentTdByLine(lineNumber, side, root);
   }
 
-  _getDiffRowByChild(child: Element) {
+  private getDiffRowByChild(child: Element) {
     while (!child.classList.contains('diff-row') && child.parentElement) {
       child = child.parentElement;
     }
@@ -348,23 +291,23 @@
     const side = getSideByLineEl(lineEl);
     // Performance optimization because we already have an element in the
     // correct row
-    const row = this._getDiffRowByChild(lineEl);
+    const row = this.getDiffRowByChild(lineEl);
     return this.getContentTdByLine(line, side, row);
   }
 
   getLineElByNumber(lineNumber: LineNumber, side?: Side) {
-    if (!this._builder) return null;
-    return this._builder.getLineElByNumber(lineNumber, side);
+    if (!this.builder) return null;
+    return this.builder.getLineElByNumber(lineNumber, side);
   }
 
   getLineNumberRows() {
-    if (!this._builder) return [];
-    return this._builder.getLineNumberRows();
+    if (!this.builder) return [];
+    return this.builder.getLineNumberRows();
   }
 
   getLineNumEls(side: Side) {
-    if (!this._builder) return [];
-    return this._builder.getLineNumEls(side);
+    if (!this.builder) return [];
+    return this.builder.getLineNumEls(side);
   }
 
   /**
@@ -376,8 +319,8 @@
    * @param side The side the line number refer to.
    */
   unhideLine(lineNum: number, side: Side) {
-    if (!this._builder) return;
-    const group = this._builder.findGroup(side, lineNum);
+    if (!this.builder) return;
+    const group = this.builder.findGroup(side, lineNum);
     // Cannot unhide a line that is not part of the diff.
     if (!group) return;
     // If it's already visible, great!
@@ -420,45 +363,50 @@
     contextGroup: GrDiffGroup,
     newGroups: readonly GrDiffGroup[]
   ) {
-    if (!this._builder) return;
-    fireEvent(this, 'render-start');
+    if (!this.builder) return;
+    this.fireDiffEvent('render-start', {});
     const linesRendered = newGroups.reduce(
       (sum, group) => sum + group.lines.length,
       0
     );
-    this._builder.replaceGroup(contextGroup, newGroups);
-    afterNextRender(this, () => {
-      fire(this, 'render-progress', {linesRendered});
-      fireEvent(this, 'render-content');
+    this.builder.replaceGroup(contextGroup, newGroups);
+    afterNextRender(() => {
+      this.fireDiffEvent('render-progress', {linesRendered});
+      this.fireDiffEvent('render-content', {});
     });
   }
 
   cancel() {
     this.processor.cancel();
-    if (this._cancelableRenderPromise) {
-      this._cancelableRenderPromise.cancel();
-      this._cancelableRenderPromise = null;
-    }
+    this.builder?.clear();
+    this.cancelableRenderPromise?.cancel();
+    this.cancelableRenderPromise = null;
+    this.diffElement?.removeEventListener(
+      'diff-context-expanded',
+      this.onDiffContextExpanded
+    );
   }
 
-  _handlePreferenceError(pref: string): never {
+  // visible for testing
+  handlePreferenceError(pref: string): never {
     const message =
       `The value of the '${pref}' user preference is ` +
       'invalid. Fix in diff preferences';
-    fireAlert(this, message);
+    assertIsDefined(this.diffElement, 'diff table');
+    fireAlert(this.diffElement, message);
     throw Error(`Invalid preference value: ${pref}`);
   }
 
-  _getDiffBuilder(): DiffBuilder {
-    if (!this.diff) {
-      throw Error('Cannot render a diff without DiffInfo.');
-    }
+  // visible for testing
+  getDiffBuilder(): DiffBuilder {
+    assertIsDefined(this.diff, 'diff');
+    assertIsDefined(this.diffElement, 'diff table');
     if (isNaN(this.prefs.tab_size) || this.prefs.tab_size <= 0) {
-      this._handlePreferenceError('tab size');
+      this.handlePreferenceError('tab size');
     }
 
     if (isNaN(this.prefs.line_length) || this.prefs.line_length <= 0) {
-      this._handlePreferenceError('diff width');
+      this.handlePreferenceError('diff width');
     }
 
     const localPrefs = {...this.prefs};
@@ -487,7 +435,7 @@
         this.diff,
         localPrefs,
         this.diffElement,
-        this._layers,
+        this.layersInternal,
         this.renderPrefs
       );
     } else if (this.viewMode === DiffViewMode.UNIFIED) {
@@ -495,7 +443,7 @@
         this.diff,
         localPrefs,
         this.diffElement,
-        this._layers,
+        this.layersInternal,
         this.renderPrefs
       );
     }
@@ -505,7 +453,8 @@
     return builder;
   }
 
-  _clearDiffContent() {
+  private clearDiffContent() {
+    assertIsDefined(this.diffElement, 'diff table');
     this.diffElement.innerHTML = '';
   }
 
@@ -514,22 +463,23 @@
    * server into chunks.
    */
   clearGroups() {
-    if (!this._builder) return;
-    this._builder.clearGroups();
+    if (!this.builder) return;
+    this.builder.clearGroups();
   }
 
   /**
    * Called when the processor is done converting a chunk of the diff.
    */
   addGroup(group: GrDiffGroup) {
-    if (!this._builder) return;
-    this._builder.addGroups([group]);
-    afterNextRender(this, () =>
-      fire(this, 'render-progress', {linesRendered: group.lines.length})
+    if (!this.builder) return;
+    this.builder.addGroups([group]);
+    afterNextRender(() =>
+      this.fireDiffEventRenderProgress({linesRendered: group.lines.length})
     );
   }
 
-  _createIntralineLayer(): DiffLayer {
+  // visible for testing
+  createIntralineLayer(): DiffLayer {
     return {
       // Take a DIV.contentText element and a line object with intraline
       // differences to highlight and apply them to the element as
@@ -561,8 +511,9 @@
     };
   }
 
-  _createTabIndicatorLayer(): DiffLayer {
-    const show = () => this._showTabs;
+  // visible for testing
+  createTabIndicatorLayer(): DiffLayer {
+    const show = () => this.showTabs;
     return {
       annotate(contentEl: HTMLElement, _: HTMLElement, line: GrDiffLine) {
         // If visible tabs are disabled, do nothing.
@@ -576,7 +527,7 @@
     };
   }
 
-  _createSpecialCharacterIndicatorLayer(): DiffLayer {
+  private createSpecialCharacterIndicatorLayer(): DiffLayer {
     return {
       annotate(contentEl: HTMLElement, _: HTMLElement, line: GrDiffLine) {
         // Find and annotate the locations of soft hyphen (\u00AD)
@@ -592,8 +543,9 @@
     };
   }
 
-  _createTrailingWhitespaceLayer(): DiffLayer {
-    const show = () => this._showTrailingWhitespace;
+  // visible for testing
+  createTrailingWhitespaceLayer(): DiffLayer {
+    const show = () => this.showTrailingWhitespace;
 
     return {
       annotate(contentEl: HTMLElement, _: HTMLElement, line: GrDiffLine) {
@@ -621,18 +573,12 @@
   }
 
   setBlame(blame: BlameInfo[] | null) {
-    if (!this._builder) return;
-    this._builder.setBlame(blame ?? []);
+    if (!this.builder) return;
+    this.builder.setBlame(blame ?? []);
   }
 
   updateRenderPrefs(renderPrefs: RenderPreferences) {
-    this._builder?.updateRenderPrefs(renderPrefs);
+    this.builder?.updateRenderPrefs(renderPrefs);
     this.processor.updateRenderPrefs(renderPrefs);
   }
 }
-
-declare global {
-  interface HTMLElementTagNameMap {
-    'gr-diff-builder': GrDiffBuilderElement;
-  }
-}
diff --git a/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-element_html.ts b/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-element_html.ts
deleted file mode 100644
index bd0e034..0000000
--- a/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-element_html.ts
+++ /dev/null
@@ -1,23 +0,0 @@
-/**
- * @license
- * Copyright (C) 2020 The Android Open Source Project
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- * http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-import {html} from '@polymer/polymer/lib/utils/html-tag';
-
-export const htmlTemplate = html`
-  <div class="contentWrapper">
-    <slot></slot>
-  </div>
-`;
diff --git a/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-element_test.js b/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-element_test.js
deleted file mode 100644
index 8c15ddd..0000000
--- a/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-element_test.js
+++ /dev/null
@@ -1,1084 +0,0 @@
-/**
- * @license
- * Copyright (C) 2016 The Android Open Source Project
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- * http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-import '../../../test/common-test-setup-karma.js';
-import {createDiff} from '../../../test/test-data-generators.js';
-import './gr-diff-builder-element.js';
-import {stubBaseUrl} from '../../../test/test-utils.js';
-import {GrAnnotation} from '../gr-diff-highlight/gr-annotation.js';
-import {GrDiffLine, GrDiffLineType} from '../gr-diff/gr-diff-line.js';
-import {GrDiffBuilderSideBySide} from './gr-diff-builder-side-by-side.js';
-import {html} from '@polymer/polymer/lib/utils/html-tag.js';
-import {DiffViewMode, Side} from '../../../api/diff.js';
-import {stubRestApi} from '../../../test/test-utils.js';
-import {afterNextRender} from '@polymer/polymer/lib/utils/render-status';
-import {GrDiffBuilderLegacy} from './gr-diff-builder-legacy.js';
-import {waitForEventOnce} from '../../../utils/event-util.js';
-
-const basicFixture = fixtureFromTemplate(html`
-    <gr-diff-builder>
-      <table id="diffTable"></table>
-    </gr-diff-builder>
-`);
-
-const divWithTextFixture = fixtureFromTemplate(html`
-<div>Lorem ipsum dolor sit amet, suspendisse inceptos vehicula</div>
-`);
-
-const mockDiffFixture = fixtureFromTemplate(html`
-<gr-diff-builder view-mode="SIDE_BY_SIDE">
-      <table id="diffTable"></table>
-    </gr-diff-builder>
-`);
-
-// GrDiffBuilderElement forces these prefs to be set - tests that do not care
-// about these values can just set these defaults.
-const DEFAULT_PREFS = {
-  line_length: 10,
-  show_tabs: true,
-  tab_size: 4,
-};
-
-suite('gr-diff-builder tests', () => {
-  let prefs;
-  let element;
-  let builder;
-
-  const LINE_BREAK_HTML = '<span class="style-scope gr-diff br"></span>';
-  const WBR_HTML = '<wbr class="style-scope gr-diff">';
-
-  setup(() => {
-    element = basicFixture.instantiate();
-    stubRestApi('getLoggedIn').returns(Promise.resolve(false));
-    stubRestApi('getProjectConfig').returns(Promise.resolve({}));
-    stubBaseUrl('/r');
-    prefs = {...DEFAULT_PREFS};
-    builder = new GrDiffBuilderLegacy({content: []}, prefs);
-  });
-
-  test('line_length applied with <wbr> if line_wrapping is true', () => {
-    builder._prefs = {line_wrapping: true, tab_size: 4, line_length: 50};
-    const text = 'a'.repeat(51);
-
-    const line = {text, highlights: []};
-    const expected = 'a'.repeat(50) + WBR_HTML + 'a';
-    const result = builder.createTextEl(undefined, line).firstChild.innerHTML;
-    assert.equal(result, expected);
-  });
-
-  test('line_length applied with line break if line_wrapping is false', () => {
-    builder._prefs = {line_wrapping: false, tab_size: 4, line_length: 50};
-    const text = 'a'.repeat(51);
-
-    const line = {text, highlights: []};
-    const expected = 'a'.repeat(50) + LINE_BREAK_HTML + 'a';
-    const result = builder.createTextEl(undefined, line).firstChild.innerHTML;
-    assert.equal(result, expected);
-  });
-
-  [DiffViewMode.UNIFIED, DiffViewMode.SIDE_BY_SIDE]
-      .forEach(mode => {
-        test(`line_length used for regular files under ${mode}`, () => {
-          element.path = '/a.txt';
-          element.viewMode = mode;
-          element.diff = {};
-          element.prefs = {tab_size: 4, line_length: 50};
-          builder = element._getDiffBuilder();
-          assert.equal(builder._prefs.line_length, 50);
-        });
-
-        test(`line_length ignored for commit msg under ${mode}`, () => {
-          element.path = '/COMMIT_MSG';
-          element.viewMode = mode;
-          element.diff = {};
-          element.prefs = {tab_size: 4, line_length: 50};
-          builder = element._getDiffBuilder();
-          assert.equal(builder._prefs.line_length, 72);
-        });
-      });
-
-  test('createTextEl linewrap with tabs', () => {
-    const text = '\t'.repeat(7) + '!';
-    const line = {text, highlights: []};
-    const el = builder.createTextEl(undefined, line);
-    assert.equal(el.innerText, text);
-    // With line length 10 and tab size 2, there should be a line break
-    // after every two tabs.
-    const newlineEl = el.querySelector('.contentText > .br');
-    assert.isOk(newlineEl);
-    assert.equal(
-        el.querySelector('.contentText .tab:nth-child(2)').nextSibling,
-        newlineEl);
-  });
-
-  test('_handlePreferenceError throws with invalid preference', () => {
-    element.prefs = {tab_size: 0};
-    assert.throws(() => element._getDiffBuilder());
-  });
-
-  test('_handlePreferenceError triggers alert and javascript error', () => {
-    const errorStub = sinon.stub();
-    element.addEventListener('show-alert', errorStub);
-    assert.throws(() => element._handlePreferenceError('tab size'));
-    assert.equal(errorStub.lastCall.args[0].detail.message,
-        `The value of the 'tab size' user preference is invalid. ` +
-      `Fix in diff preferences`);
-  });
-
-  suite('intraline differences', () => {
-    let el;
-    let str;
-    let annotateElementSpy;
-    let layer;
-    const lineNumberEl = document.createElement('td');
-
-    function slice(str, start, end) {
-      return Array.from(str).slice(start, end)
-          .join('');
-    }
-
-    setup(() => {
-      el = divWithTextFixture.instantiate();
-      str = el.textContent;
-      annotateElementSpy = sinon.spy(GrAnnotation, 'annotateElement');
-      layer = document.createElement('gr-diff-builder')
-          ._createIntralineLayer();
-    });
-
-    test('annotate no highlights', () => {
-      const line = {
-        text: str,
-        highlights: [],
-      };
-
-      layer.annotate(el, lineNumberEl, line);
-
-      // The content is unchanged.
-      assert.isFalse(annotateElementSpy.called);
-      assert.equal(el.childNodes.length, 1);
-      assert.instanceOf(el.childNodes[0], Text);
-      assert.equal(str, el.childNodes[0].textContent);
-    });
-
-    test('annotate with highlights', () => {
-      const line = {
-        text: str,
-        highlights: [
-          {startIndex: 6, endIndex: 12},
-          {startIndex: 18, endIndex: 22},
-        ],
-      };
-      const str0 = slice(str, 0, 6);
-      const str1 = slice(str, 6, 12);
-      const str2 = slice(str, 12, 18);
-      const str3 = slice(str, 18, 22);
-      const str4 = slice(str, 22);
-
-      layer.annotate(el, lineNumberEl, line);
-
-      assert.isTrue(annotateElementSpy.called);
-      assert.equal(el.childNodes.length, 5);
-
-      assert.instanceOf(el.childNodes[0], Text);
-      assert.equal(el.childNodes[0].textContent, str0);
-
-      assert.notInstanceOf(el.childNodes[1], Text);
-      assert.equal(el.childNodes[1].textContent, str1);
-
-      assert.instanceOf(el.childNodes[2], Text);
-      assert.equal(el.childNodes[2].textContent, str2);
-
-      assert.notInstanceOf(el.childNodes[3], Text);
-      assert.equal(el.childNodes[3].textContent, str3);
-
-      assert.instanceOf(el.childNodes[4], Text);
-      assert.equal(el.childNodes[4].textContent, str4);
-    });
-
-    test('annotate without endIndex', () => {
-      const line = {
-        text: str,
-        highlights: [
-          {startIndex: 28},
-        ],
-      };
-
-      const str0 = slice(str, 0, 28);
-      const str1 = slice(str, 28);
-
-      layer.annotate(el, lineNumberEl, line);
-
-      assert.isTrue(annotateElementSpy.called);
-      assert.equal(el.childNodes.length, 2);
-
-      assert.instanceOf(el.childNodes[0], Text);
-      assert.equal(el.childNodes[0].textContent, str0);
-
-      assert.notInstanceOf(el.childNodes[1], Text);
-      assert.equal(el.childNodes[1].textContent, str1);
-    });
-
-    test('annotate ignores empty highlights', () => {
-      const line = {
-        text: str,
-        highlights: [
-          {startIndex: 28, endIndex: 28},
-        ],
-      };
-
-      layer.annotate(el, lineNumberEl, line);
-
-      assert.isFalse(annotateElementSpy.called);
-      assert.equal(el.childNodes.length, 1);
-    });
-
-    test('annotate handles unicode', () => {
-      // Put some unicode into the string:
-      str = str.replace(/\s/g, '💢');
-      el.textContent = str;
-      const line = {
-        text: str,
-        highlights: [
-          {startIndex: 6, endIndex: 12},
-        ],
-      };
-
-      const str0 = slice(str, 0, 6);
-      const str1 = slice(str, 6, 12);
-      const str2 = slice(str, 12);
-
-      layer.annotate(el, lineNumberEl, line);
-
-      assert.isTrue(annotateElementSpy.called);
-      assert.equal(el.childNodes.length, 3);
-
-      assert.instanceOf(el.childNodes[0], Text);
-      assert.equal(el.childNodes[0].textContent, str0);
-
-      assert.notInstanceOf(el.childNodes[1], Text);
-      assert.equal(el.childNodes[1].textContent, str1);
-
-      assert.instanceOf(el.childNodes[2], Text);
-      assert.equal(el.childNodes[2].textContent, str2);
-    });
-
-    test('annotate handles unicode w/o endIndex', () => {
-      // Put some unicode into the string:
-      str = str.replace(/\s/g, '💢');
-      el.textContent = str;
-
-      const line = {
-        text: str,
-        highlights: [
-          {startIndex: 6},
-        ],
-      };
-
-      const str0 = slice(str, 0, 6);
-      const str1 = slice(str, 6);
-
-      layer.annotate(el, lineNumberEl, line);
-
-      assert.isTrue(annotateElementSpy.called);
-      assert.equal(el.childNodes.length, 2);
-
-      assert.instanceOf(el.childNodes[0], Text);
-      assert.equal(el.childNodes[0].textContent, str0);
-
-      assert.notInstanceOf(el.childNodes[1], Text);
-      assert.equal(el.childNodes[1].textContent, str1);
-    });
-  });
-
-  suite('tab indicators', () => {
-    let element;
-    let layer;
-    const lineNumberEl = document.createElement('td');
-
-    setup(() => {
-      element = basicFixture.instantiate();
-      element._showTabs = true;
-      layer = element._createTabIndicatorLayer();
-    });
-
-    test('does nothing with empty line', () => {
-      const line = {text: ''};
-      const el = document.createElement('div');
-      const annotateElementStub =
-          sinon.stub(GrAnnotation, 'annotateElement');
-
-      layer.annotate(el, lineNumberEl, line);
-
-      assert.isFalse(annotateElementStub.called);
-    });
-
-    test('does nothing with no tabs', () => {
-      const str = 'lorem ipsum no tabs';
-      const line = {text: str};
-      const el = document.createElement('div');
-      el.textContent = str;
-      const annotateElementStub =
-          sinon.stub(GrAnnotation, 'annotateElement');
-
-      layer.annotate(el, lineNumberEl, line);
-
-      assert.isFalse(annotateElementStub.called);
-    });
-
-    test('annotates tab at beginning', () => {
-      const str = '\tlorem upsum';
-      const line = {text: str};
-      const el = document.createElement('div');
-      el.textContent = str;
-      const annotateElementStub =
-          sinon.stub(GrAnnotation, 'annotateElement');
-
-      layer.annotate(el, lineNumberEl, line);
-
-      assert.equal(annotateElementStub.callCount, 1);
-      const args = annotateElementStub.getCalls()[0].args;
-      assert.equal(args[0], el);
-      assert.equal(args[1], 0, 'offset of tab indicator');
-      assert.equal(args[2], 1, 'length of tab indicator');
-      assert.include(args[3], 'tab-indicator');
-    });
-
-    test('does not annotate when disabled', () => {
-      element._showTabs = false;
-
-      const str = '\tlorem upsum';
-      const line = {text: str};
-      const el = document.createElement('div');
-      el.textContent = str;
-      const annotateElementStub =
-          sinon.stub(GrAnnotation, 'annotateElement');
-
-      layer.annotate(el, lineNumberEl, line);
-
-      assert.isFalse(annotateElementStub.called);
-    });
-
-    test('annotates multiple in beginning', () => {
-      const str = '\t\tlorem upsum';
-      const line = {text: str};
-      const el = document.createElement('div');
-      el.textContent = str;
-      const annotateElementStub =
-          sinon.stub(GrAnnotation, 'annotateElement');
-
-      layer.annotate(el, lineNumberEl, line);
-
-      assert.equal(annotateElementStub.callCount, 2);
-
-      let args = annotateElementStub.getCalls()[0].args;
-      assert.equal(args[0], el);
-      assert.equal(args[1], 0, 'offset of tab indicator');
-      assert.equal(args[2], 1, 'length of tab indicator');
-      assert.include(args[3], 'tab-indicator');
-
-      args = annotateElementStub.getCalls()[1].args;
-      assert.equal(args[0], el);
-      assert.equal(args[1], 1, 'offset of tab indicator');
-      assert.equal(args[2], 1, 'length of tab indicator');
-      assert.include(args[3], 'tab-indicator');
-    });
-
-    test('annotates intermediate tabs', () => {
-      const str = 'lorem\tupsum';
-      const line = {text: str};
-      const el = document.createElement('div');
-      el.textContent = str;
-      const annotateElementStub =
-          sinon.stub(GrAnnotation, 'annotateElement');
-
-      layer.annotate(el, lineNumberEl, line);
-
-      assert.equal(annotateElementStub.callCount, 1);
-      const args = annotateElementStub.getCalls()[0].args;
-      assert.equal(args[0], el);
-      assert.equal(args[1], 5, 'offset of tab indicator');
-      assert.equal(args[2], 1, 'length of tab indicator');
-      assert.include(args[3], 'tab-indicator');
-    });
-  });
-
-  suite('layers', () => {
-    let element;
-    let initialLayersCount;
-    let withLayerCount;
-    setup(() => {
-      const layers = [];
-      element = basicFixture.instantiate();
-      element.layers = layers;
-      element._showTrailingWhitespace = true;
-      element._setupAnnotationLayers();
-      initialLayersCount = element._layers.length;
-    });
-
-    test('no layers', () => {
-      element._setupAnnotationLayers();
-      assert.equal(element._layers.length, initialLayersCount);
-    });
-
-    suite('with layers', () => {
-      const layers = [{}, {}];
-      setup(() => {
-        element = basicFixture.instantiate();
-        element.layers = layers;
-        element._showTrailingWhitespace = true;
-        element._setupAnnotationLayers();
-        withLayerCount = element._layers.length;
-      });
-      test('with layers', () => {
-        element._setupAnnotationLayers();
-        assert.equal(element._layers.length, withLayerCount);
-        assert.equal(initialLayersCount + layers.length,
-            withLayerCount);
-      });
-    });
-  });
-
-  suite('trailing whitespace', () => {
-    let element;
-    let layer;
-    const lineNumberEl = document.createElement('td');
-
-    setup(() => {
-      element = basicFixture.instantiate();
-      element._showTrailingWhitespace = true;
-      layer = element._createTrailingWhitespaceLayer();
-    });
-
-    test('does nothing with empty line', () => {
-      const line = {text: ''};
-      const el = document.createElement('div');
-      const annotateElementStub =
-          sinon.stub(GrAnnotation, 'annotateElement');
-      layer.annotate(el, lineNumberEl, line);
-      assert.isFalse(annotateElementStub.called);
-    });
-
-    test('does nothing with no trailing whitespace', () => {
-      const str = 'lorem ipsum blah blah';
-      const line = {text: str};
-      const el = document.createElement('div');
-      el.textContent = str;
-      const annotateElementStub =
-          sinon.stub(GrAnnotation, 'annotateElement');
-      layer.annotate(el, lineNumberEl, line);
-      assert.isFalse(annotateElementStub.called);
-    });
-
-    test('annotates trailing spaces', () => {
-      const str = 'lorem ipsum   ';
-      const line = {text: str};
-      const el = document.createElement('div');
-      el.textContent = str;
-      const annotateElementStub =
-          sinon.stub(GrAnnotation, 'annotateElement');
-      layer.annotate(el, lineNumberEl, line);
-      assert.isTrue(annotateElementStub.called);
-      assert.equal(annotateElementStub.lastCall.args[1], 11);
-      assert.equal(annotateElementStub.lastCall.args[2], 3);
-    });
-
-    test('annotates trailing tabs', () => {
-      const str = 'lorem ipsum\t\t\t';
-      const line = {text: str};
-      const el = document.createElement('div');
-      el.textContent = str;
-      const annotateElementStub =
-          sinon.stub(GrAnnotation, 'annotateElement');
-      layer.annotate(el, lineNumberEl, line);
-      assert.isTrue(annotateElementStub.called);
-      assert.equal(annotateElementStub.lastCall.args[1], 11);
-      assert.equal(annotateElementStub.lastCall.args[2], 3);
-    });
-
-    test('annotates mixed trailing whitespace', () => {
-      const str = 'lorem ipsum\t \t';
-      const line = {text: str};
-      const el = document.createElement('div');
-      el.textContent = str;
-      const annotateElementStub =
-          sinon.stub(GrAnnotation, 'annotateElement');
-      layer.annotate(el, lineNumberEl, line);
-      assert.isTrue(annotateElementStub.called);
-      assert.equal(annotateElementStub.lastCall.args[1], 11);
-      assert.equal(annotateElementStub.lastCall.args[2], 3);
-    });
-
-    test('unicode preceding trailing whitespace', () => {
-      const str = '💢\t';
-      const line = {text: str};
-      const el = document.createElement('div');
-      el.textContent = str;
-      const annotateElementStub =
-          sinon.stub(GrAnnotation, 'annotateElement');
-      layer.annotate(el, lineNumberEl, line);
-      assert.isTrue(annotateElementStub.called);
-      assert.equal(annotateElementStub.lastCall.args[1], 1);
-      assert.equal(annotateElementStub.lastCall.args[2], 1);
-    });
-
-    test('does not annotate when disabled', () => {
-      element._showTrailingWhitespace = false;
-      const str = 'lorem upsum\t \t ';
-      const line = {text: str};
-      const el = document.createElement('div');
-      el.textContent = str;
-      const annotateElementStub =
-          sinon.stub(GrAnnotation, 'annotateElement');
-      layer.annotate(el, lineNumberEl, line);
-      assert.isFalse(annotateElementStub.called);
-    });
-  });
-
-  suite('rendering text, images and binary files', () => {
-    let processStub;
-    let keyLocations;
-    let content;
-
-    setup(() => {
-      element = basicFixture.instantiate();
-      element.viewMode = 'SIDE_BY_SIDE';
-      processStub = sinon.stub(element.processor, 'process')
-          .returns(Promise.resolve());
-      keyLocations = {left: {}, right: {}};
-      element.prefs = {
-        ...DEFAULT_PREFS,
-        context: -1,
-        syntax_highlighting: true,
-      };
-      content = [{
-        a: ['all work and no play make andybons a dull boy'],
-        b: ['elgoog elgoog elgoog'],
-      }, {
-        ab: [
-          'Non eram nescius, Brute, cum, quae summis ingeniis ',
-          'exquisitaque doctrina philosophi Graeco sermone tractavissent',
-        ],
-      }];
-    });
-
-    test('text', async () => {
-      element.diff = {content};
-      element.render(keyLocations);
-      await waitForEventOnce(element, 'render-content');
-      assert.isTrue(processStub.calledOnce);
-      assert.isFalse(processStub.lastCall.args[1]);
-    });
-
-    test('image', async () => {
-      element.diff = {content, binary: true};
-      element.isImageDiff = true;
-      element.render(keyLocations);
-      await waitForEventOnce(element, 'render-content');
-      assert.isTrue(processStub.calledOnce);
-      assert.isTrue(processStub.lastCall.args[1]);
-    });
-
-    test('binary', async () => {
-      element.diff = {content, binary: true};
-      element.render(keyLocations);
-      await waitForEventOnce(element, 'render-content');
-      assert.isTrue(processStub.calledOnce);
-      assert.isTrue(processStub.lastCall.args[1]);
-    });
-  });
-
-  suite('rendering', () => {
-    let content;
-    let outputEl;
-    let keyLocations;
-
-    setup(async () => {
-      const prefs = {...DEFAULT_PREFS};
-      content = [
-        {
-          a: ['all work and no play make andybons a dull boy'],
-          b: ['elgoog elgoog elgoog'],
-        },
-        {
-          ab: [
-            'Non eram nescius, Brute, cum, quae summis ingeniis ',
-            'exquisitaque doctrina philosophi Graeco sermone tractavissent',
-          ],
-        },
-      ];
-      element = basicFixture.instantiate();
-      sinon.stub(element, 'dispatchEvent');
-      outputEl = element.querySelector('#diffTable');
-      keyLocations = {left: {}, right: {}};
-      sinon.stub(element, '_getDiffBuilder').callsFake(() => {
-        const builder = new GrDiffBuilderSideBySide({content}, prefs, outputEl);
-        sinon.stub(builder, 'addColumns');
-        builder.buildSectionElement = function(group) {
-          const section = document.createElement('stub');
-          section.textContent = group.lines
-              .reduce((acc, line) => acc + line.text, '');
-          return section;
-        };
-        return builder;
-      });
-      element.diff = {content};
-      element.prefs = prefs;
-      await element.render(keyLocations);
-    });
-
-    test('addColumns is called', () => {
-      assert.isTrue(element._builder.addColumns.called);
-    });
-
-    test('getGroupsByLineRange one line', () => {
-      const section = outputEl.querySelector('stub:nth-of-type(3)');
-      const groups = element._builder.getGroupsByLineRange(1, 1, 'left');
-      assert.equal(groups.length, 1);
-      assert.strictEqual(groups[0].element, section);
-    });
-
-    test('getGroupsByLineRange over diff', () => {
-      const section = [
-        outputEl.querySelector('stub:nth-of-type(3)'),
-        outputEl.querySelector('stub:nth-of-type(4)'),
-      ];
-      const groups = element._builder.getGroupsByLineRange(1, 2, 'left');
-      assert.equal(groups.length, 2);
-      assert.strictEqual(groups[0].element, section[0]);
-      assert.strictEqual(groups[1].element, section[1]);
-    });
-
-    test('render-start and render-content are fired', async () => {
-      await new Promise(resolve => afterNextRender(element, resolve));
-      const firedEventTypes = element.dispatchEvent.getCalls()
-          .map(c => c.args[0].type);
-      assert.include(firedEventTypes, 'render-start');
-      assert.include(firedEventTypes, 'render-content');
-    });
-
-    test('cancel cancels the processor', () => {
-      const processorCancelStub = sinon.stub(element.processor, 'cancel');
-      element.cancel();
-      assert.isTrue(processorCancelStub.called);
-    });
-  });
-
-  suite('context hiding and expanding', () => {
-    setup(async () => {
-      element = basicFixture.instantiate();
-      sinon.stub(element, 'dispatchEvent');
-      const afterNextRenderPromise = new Promise((resolve, reject) => {
-        afterNextRender(element, resolve);
-      });
-      element.diff = {
-        content: [
-          {ab: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10].map(i => `unchanged ${i}`)},
-          {a: ['before'], b: ['after']},
-          {ab: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10].map(i => `unchanged ${10 + i}`)},
-        ],
-      };
-      element.viewMode = DiffViewMode.SIDE_BY_SIDE;
-
-      const keyLocations = {left: {}, right: {}};
-      element.prefs = {
-        ...DEFAULT_PREFS,
-        context: 1,
-      };
-      await element.render(keyLocations);
-      // Make sure all listeners are installed.
-      await afterNextRenderPromise;
-    });
-
-    test('hides lines behind two context controls', () => {
-      const contextControls = element.querySelectorAll('gr-context-controls');
-      assert.equal(contextControls.length, 2);
-
-      const diffRows = element.querySelectorAll('.diff-row');
-      // The first two are LOST and FILE line
-      assert.equal(diffRows.length, 2 + 1 + 1 + 1);
-      assert.include(diffRows[2].textContent, 'unchanged 10');
-      assert.include(diffRows[3].textContent, 'before');
-      assert.include(diffRows[3].textContent, 'after');
-      assert.include(diffRows[4].textContent, 'unchanged 11');
-    });
-
-    test('clicking +x common lines expands those lines', () => {
-      const contextControls = element.querySelectorAll('gr-context-controls');
-      const topExpandCommonButton = contextControls[0].shadowRoot
-          .querySelectorAll('.showContext')[0];
-      assert.include(topExpandCommonButton.textContent, '+9 common lines');
-      topExpandCommonButton.click();
-      const diffRows = element.querySelectorAll('.diff-row');
-      // The first two are LOST and FILE line
-      assert.equal(diffRows.length, 2 + 10 + 1 + 1);
-      assert.include(diffRows[2].textContent, 'unchanged 1');
-      assert.include(diffRows[3].textContent, 'unchanged 2');
-      assert.include(diffRows[4].textContent, 'unchanged 3');
-      assert.include(diffRows[5].textContent, 'unchanged 4');
-      assert.include(diffRows[6].textContent, 'unchanged 5');
-      assert.include(diffRows[7].textContent, 'unchanged 6');
-      assert.include(diffRows[8].textContent, 'unchanged 7');
-      assert.include(diffRows[9].textContent, 'unchanged 8');
-      assert.include(diffRows[10].textContent, 'unchanged 9');
-      assert.include(diffRows[11].textContent, 'unchanged 10');
-      assert.include(diffRows[12].textContent, 'before');
-      assert.include(diffRows[12].textContent, 'after');
-      assert.include(diffRows[13].textContent, 'unchanged 11');
-    });
-
-    test('unhideLine shows the line with context', async () => {
-      element.dispatchEvent.reset();
-      element.unhideLine(4, Side.LEFT);
-
-      const diffRows = element.querySelectorAll('.diff-row');
-      // The first two are LOST and FILE line
-      // Lines 3-5 (Line 4 plus 1 context in each direction) will be expanded
-      // Because context expanders do not hide <3 lines, lines 1-2 will also
-      // be shown.
-      // Lines 6-9 continue to be hidden
-      assert.equal(diffRows.length, 2 + 5 + 1 + 1 + 1);
-      assert.include(diffRows[2].textContent, 'unchanged 1');
-      assert.include(diffRows[3].textContent, 'unchanged 2');
-      assert.include(diffRows[4].textContent, 'unchanged 3');
-      assert.include(diffRows[5].textContent, 'unchanged 4');
-      assert.include(diffRows[6].textContent, 'unchanged 5');
-      assert.include(diffRows[7].textContent, 'unchanged 10');
-      assert.include(diffRows[8].textContent, 'before');
-      assert.include(diffRows[8].textContent, 'after');
-      assert.include(diffRows[9].textContent, 'unchanged 11');
-
-      await new Promise(resolve => afterNextRender(element, resolve));
-      const firedEventTypes = element.dispatchEvent.getCalls()
-          .map(c => c.args[0].type);
-      assert.include(firedEventTypes, 'render-content');
-    });
-  });
-
-  suite('mock-diff', () => {
-    let element;
-    let builder;
-    let diff;
-    let keyLocations;
-
-    setup(async () => {
-      element = mockDiffFixture.instantiate();
-      diff = createDiff();
-      element.diff = diff;
-
-      keyLocations = {left: {}, right: {}};
-
-      element.prefs = {
-        line_length: 80,
-        show_tabs: true,
-        tab_size: 4,
-      };
-      await element.render(keyLocations);
-      builder = element._builder;
-    });
-
-    test('aria-labels on added line numbers', () => {
-      const deltaLineNumberButton = element.diffElement.querySelectorAll(
-          '.lineNumButton.right')[5];
-
-      assert.isOk(deltaLineNumberButton);
-      assert.equal(deltaLineNumberButton.getAttribute('aria-label'), '5 added');
-    });
-
-    test('aria-labels on removed line numbers', () => {
-      const deltaLineNumberButton = element.diffElement.querySelectorAll(
-          '.lineNumButton.left')[10];
-
-      assert.isOk(deltaLineNumberButton);
-      assert.equal(
-          deltaLineNumberButton.getAttribute('aria-label'), '10 removed');
-    });
-
-    test('getContentByLine', () => {
-      let actual;
-
-      actual = builder.getContentByLine(2, 'left');
-      assert.equal(actual.textContent, diff.content[0].ab[1]);
-
-      actual = builder.getContentByLine(2, 'right');
-      assert.equal(actual.textContent, diff.content[0].ab[1]);
-
-      actual = builder.getContentByLine(5, 'left');
-      assert.equal(actual.textContent, diff.content[2].ab[0]);
-
-      actual = builder.getContentByLine(5, 'right');
-      assert.equal(actual.textContent, diff.content[1].b[0]);
-    });
-
-    test('getContentTdByLineEl works both with button and td', () => {
-      const diffRow = element.diffElement.querySelectorAll('tr.diff-row')[2];
-
-      const lineNumTdLeft = diffRow.querySelector('td.lineNum.left');
-      const lineNumButtonLeft = lineNumTdLeft.querySelector('button');
-      const contentTdLeft = diffRow.querySelectorAll('.content')[0];
-
-      const lineNumTdRight = diffRow.querySelector('td.lineNum.right');
-      const lineNumButtonRight = lineNumTdRight.querySelector('button');
-      const contentTdRight = diffRow.querySelectorAll('.content')[1];
-
-      assert.equal(element.getContentTdByLineEl(lineNumTdLeft), contentTdLeft);
-      assert.equal(
-          element.getContentTdByLineEl(lineNumButtonLeft), contentTdLeft);
-      assert.equal(
-          element.getContentTdByLineEl(lineNumTdRight), contentTdRight);
-      assert.equal(
-          element.getContentTdByLineEl(lineNumButtonRight), contentTdRight);
-    });
-
-    test('findLinesByRange', () => {
-      const lines = [];
-      const elems = [];
-      const start = 6;
-      const end = 10;
-      const count = end - start + 1;
-
-      builder.findLinesByRange(start, end, 'right', lines, elems);
-
-      assert.equal(lines.length, count);
-      assert.equal(elems.length, count);
-
-      for (let i = 0; i < 5; i++) {
-        assert.instanceOf(lines[i], GrDiffLine);
-        assert.equal(lines[i].afterNumber, start + i);
-        assert.instanceOf(elems[i], HTMLElement);
-        assert.equal(lines[i].text, elems[i].textContent);
-      }
-    });
-
-    test('renderContentByRange', () => {
-      const spy = sinon.spy(builder, 'createTextEl');
-      const start = 9;
-      const end = 14;
-      const count = end - start + 1;
-
-      builder.renderContentByRange(start, end, 'left');
-
-      assert.equal(spy.callCount, count);
-      spy.getCalls().forEach((call, i) => {
-        assert.equal(call.args[1].beforeNumber, start + i);
-      });
-    });
-
-    test('renderContentByRange non-existent elements', () => {
-      const spy = sinon.spy(builder, 'createTextEl');
-
-      sinon.stub(builder, 'getLineNumberEl').returns(
-          document.createElement('div')
-      );
-      sinon.stub(builder, 'findLinesByRange').callsFake(
-          (s, e, d, lines, elements) => {
-            // Add a line and a corresponding element.
-            lines.push(new GrDiffLine(GrDiffLineType.BOTH));
-            const tr = document.createElement('tr');
-            const td = document.createElement('td');
-            const el = document.createElement('div');
-            tr.appendChild(td);
-            td.appendChild(el);
-            elements.push(el);
-
-            // Add 2 lines without corresponding elements.
-            lines.push(new GrDiffLine(GrDiffLineType.BOTH));
-            lines.push(new GrDiffLine(GrDiffLineType.BOTH));
-          });
-
-      builder.renderContentByRange(1, 10, 'left');
-      // Should be called only once because only one line had a corresponding
-      // element.
-      assert.equal(spy.callCount, 1);
-    });
-
-    test('getLineNumberEl side-by-side left', () => {
-      const contentEl = builder.getContentByLine(5, 'left',
-          element.$.diffTable);
-      const lineNumberEl = builder.getLineNumberEl(contentEl, 'left');
-      assert.isTrue(lineNumberEl.classList.contains('lineNum'));
-      assert.isTrue(lineNumberEl.classList.contains('left'));
-    });
-
-    test('getLineNumberEl side-by-side right', () => {
-      const contentEl = builder.getContentByLine(5, 'right',
-          element.$.diffTable);
-      const lineNumberEl = builder.getLineNumberEl(contentEl, 'right');
-      assert.isTrue(lineNumberEl.classList.contains('lineNum'));
-      assert.isTrue(lineNumberEl.classList.contains('right'));
-    });
-
-    test('getLineNumberEl unified left', async () => {
-      // Re-render as unified:
-      element.viewMode = 'UNIFIED_DIFF';
-      await element.render(keyLocations);
-      builder = element._builder;
-
-      const contentEl = builder.getContentByLine(5, 'left',
-          element.$.diffTable);
-      const lineNumberEl = builder.getLineNumberEl(contentEl, 'left');
-      assert.isTrue(lineNumberEl.classList.contains('lineNum'));
-      assert.isTrue(lineNumberEl.classList.contains('left'));
-    });
-
-    test('getLineNumberEl unified right', async () => {
-      // Re-render as unified:
-      element.viewMode = 'UNIFIED_DIFF';
-      await element.render(keyLocations);
-      builder = element._builder;
-
-      const contentEl = builder.getContentByLine(5, 'right',
-          element.$.diffTable);
-      const lineNumberEl = builder.getLineNumberEl(contentEl, 'right');
-      assert.isTrue(lineNumberEl.classList.contains('lineNum'));
-      assert.isTrue(lineNumberEl.classList.contains('right'));
-    });
-
-    test('getNextContentOnSide side-by-side left', () => {
-      const startElem = builder.getContentByLine(5, 'left',
-          element.$.diffTable);
-      const expectedStartString = diff.content[2].ab[0];
-      const expectedNextString = diff.content[2].ab[1];
-      assert.equal(startElem.textContent, expectedStartString);
-
-      const nextElem = builder.getNextContentOnSide(startElem,
-          'left');
-      assert.equal(nextElem.textContent, expectedNextString);
-    });
-
-    test('getNextContentOnSide side-by-side right', () => {
-      const startElem = builder.getContentByLine(5, 'right',
-          element.$.diffTable);
-      const expectedStartString = diff.content[1].b[0];
-      const expectedNextString = diff.content[1].b[1];
-      assert.equal(startElem.textContent, expectedStartString);
-
-      const nextElem = builder.getNextContentOnSide(startElem,
-          'right');
-      assert.equal(nextElem.textContent, expectedNextString);
-    });
-
-    test('getNextContentOnSide unified left', async () => {
-      // Re-render as unified:
-      element.viewMode = 'UNIFIED_DIFF';
-      await element.render(keyLocations);
-      builder = element._builder;
-
-      const startElem = builder.getContentByLine(5, 'left',
-          element.$.diffTable);
-      const expectedStartString = diff.content[2].ab[0];
-      const expectedNextString = diff.content[2].ab[1];
-      assert.equal(startElem.textContent, expectedStartString);
-
-      const nextElem = builder.getNextContentOnSide(startElem,
-          'left');
-      assert.equal(nextElem.textContent, expectedNextString);
-    });
-
-    test('getNextContentOnSide unified right', async () => {
-      // Re-render as unified:
-      element.viewMode = 'UNIFIED_DIFF';
-      await element.render(keyLocations);
-      builder = element._builder;
-
-      const startElem = builder.getContentByLine(5, 'right',
-          element.$.diffTable);
-      const expectedStartString = diff.content[1].b[0];
-      const expectedNextString = diff.content[1].b[1];
-      assert.equal(startElem.textContent, expectedStartString);
-
-      const nextElem = builder.getNextContentOnSide(startElem,
-          'right');
-      assert.equal(nextElem.textContent, expectedNextString);
-    });
-  });
-
-  suite('blame', () => {
-    let mockBlame;
-
-    setup(() => {
-      mockBlame = [
-        {id: 'commit 1', ranges: [{start: 1, end: 2}, {start: 10, end: 16}]},
-        {id: 'commit 2', ranges: [{start: 4, end: 10}, {start: 17, end: 32}]},
-      ];
-    });
-
-    test('setBlame attempts to render each blamed line', () => {
-      const getBlameStub = sinon.stub(builder, 'getBlameTdByLine')
-          .returns(null);
-      builder.setBlame(mockBlame);
-      assert.equal(getBlameStub.callCount, 32);
-    });
-
-    test('getBlameCommitForBaseLine', () => {
-      sinon.stub(builder, 'getBlameTdByLine').returns(undefined);
-      builder.setBlame(mockBlame);
-      assert.isOk(builder.getBlameCommitForBaseLine(1));
-      assert.equal(builder.getBlameCommitForBaseLine(1).id, 'commit 1');
-
-      assert.isOk(builder.getBlameCommitForBaseLine(11));
-      assert.equal(builder.getBlameCommitForBaseLine(11).id, 'commit 1');
-
-      assert.isOk(builder.getBlameCommitForBaseLine(32));
-      assert.equal(builder.getBlameCommitForBaseLine(32).id, 'commit 2');
-
-      assert.isUndefined(builder.getBlameCommitForBaseLine(33));
-    });
-
-    test('getBlameCommitForBaseLine w/o blame returns null', () => {
-      assert.isUndefined(builder.getBlameCommitForBaseLine(1));
-      assert.isUndefined(builder.getBlameCommitForBaseLine(11));
-      assert.isUndefined(builder.getBlameCommitForBaseLine(31));
-    });
-
-    test('createBlameCell', () => {
-      const mockBlameInfo = {
-        time: 1576155200,
-        id: 1234567890,
-        author: 'Clark Kent',
-        commit_msg: 'Testing Commit',
-        ranges: [1],
-      };
-      const getBlameStub = sinon.stub(builder, 'getBlameCommitForBaseLine')
-          .returns(mockBlameInfo);
-      const line = new GrDiffLine(GrDiffLineType.BOTH);
-      line.beforeNumber = 3;
-      line.afterNumber = 5;
-
-      const result = builder.createBlameCell(line.beforeNumber);
-
-      assert.isTrue(getBlameStub.calledWithExactly(3));
-      assert.equal(result.getAttribute('data-line-number'), '3');
-      expect(result).dom.to.equal(/* HTML */`
-        <span class="gr-diff style-scope">
-          <a class="blameDate gr-diff style-scope" href="/r/q/1234567890">
-            12/12/2019
-          </a>
-          <span class="blameAuthor gr-diff style-scope">Clark</span>
-          <gr-hovercard class="gr-diff style-scope">
-            <span class="blameHoverCard gr-diff style-scope">
-              Commit 1234567890<br>
-              Author: Clark Kent<br>
-              Date: 12/12/2019<br>
-              <br>
-              Testing Commit
-            </span>
-          </gr-hovercard>
-        </span>
-      `);
-    });
-  });
-});
-
diff --git a/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-element_test.ts b/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-element_test.ts
new file mode 100644
index 0000000..8ae08f4
--- /dev/null
+++ b/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-element_test.ts
@@ -0,0 +1,1131 @@
+/**
+ * @license
+ * Copyright 2016 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+import '../../../test/common-test-setup-karma';
+import {
+  createConfig,
+  createDiff,
+  createEmptyDiff,
+} from '../../../test/test-data-generators';
+import './gr-diff-builder-element';
+import {
+  nextRender,
+  queryAndAssert,
+  stubBaseUrl,
+} from '../../../test/test-utils';
+import {GrAnnotation} from '../gr-diff-highlight/gr-annotation';
+import {GrDiffLine, GrDiffLineType} from '../gr-diff/gr-diff-line';
+import {GrDiffBuilderSideBySide} from './gr-diff-builder-side-by-side';
+import {
+  DiffContent,
+  DiffInfo,
+  DiffLayer,
+  DiffPreferencesInfo,
+  DiffViewMode,
+  Side,
+} from '../../../api/diff';
+import {stubRestApi} from '../../../test/test-utils';
+import {GrDiffBuilderLegacy} from './gr-diff-builder-legacy';
+import {waitForEventOnce} from '../../../utils/event-util';
+import {GrDiffBuilderElement} from './gr-diff-builder-element';
+import {createDefaultDiffPrefs} from '../../../constants/constants';
+import {KeyLocations} from '../gr-diff-processor/gr-diff-processor';
+import {BlameInfo} from '../../../types/common';
+import {fixture, html} from '@open-wc/testing-helpers';
+
+const DEFAULT_PREFS = createDefaultDiffPrefs();
+
+suite('gr-diff-builder tests', () => {
+  let element: GrDiffBuilderElement;
+  let builder: GrDiffBuilderLegacy;
+  let diffTable: HTMLTableElement;
+
+  const LINE_BREAK_HTML = '<span class="style-scope gr-diff br"></span>';
+  const WBR_HTML = '<wbr class="style-scope gr-diff">';
+
+  const setBuilderPrefs = (prefs: Partial<DiffPreferencesInfo>) => {
+    builder = new GrDiffBuilderSideBySide(
+      createEmptyDiff(),
+      {...createDefaultDiffPrefs(), ...prefs},
+      diffTable
+    );
+  };
+
+  const line = (text: string) => {
+    const line = new GrDiffLine(GrDiffLineType.BOTH);
+    line.text = text;
+    return line;
+  };
+
+  setup(async () => {
+    diffTable = await fixture(html`<table id="diffTable"></table>`);
+    element = new GrDiffBuilderElement();
+    element.diffElement = diffTable;
+    stubRestApi('getLoggedIn').returns(Promise.resolve(false));
+    stubRestApi('getProjectConfig').returns(Promise.resolve(createConfig()));
+    stubBaseUrl('/r');
+    setBuilderPrefs({});
+  });
+
+  test('line_length applied with <wbr> if line_wrapping is true', () => {
+    setBuilderPrefs({line_wrapping: true, tab_size: 4, line_length: 50});
+    const text = 'a'.repeat(51);
+    const expected = 'a'.repeat(50) + WBR_HTML + 'a';
+    const result = builder.createTextEl(null, line(text)).firstElementChild
+      ?.innerHTML;
+    assert.equal(result, expected);
+  });
+
+  test('line_length applied with line break if line_wrapping is false', () => {
+    setBuilderPrefs({line_wrapping: false, tab_size: 4, line_length: 50});
+    const text = 'a'.repeat(51);
+    const expected = 'a'.repeat(50) + LINE_BREAK_HTML + 'a';
+    const result = builder.createTextEl(null, line(text)).firstElementChild
+      ?.innerHTML;
+    assert.equal(result, expected);
+  });
+
+  [DiffViewMode.UNIFIED, DiffViewMode.SIDE_BY_SIDE].forEach(mode => {
+    test(`line_length used for regular files under ${mode}`, () => {
+      element.path = '/a.txt';
+      element.viewMode = mode;
+      element.diff = createEmptyDiff();
+      element.prefs = {
+        ...createDefaultDiffPrefs(),
+        tab_size: 4,
+        line_length: 50,
+      };
+      builder = element.getDiffBuilder() as GrDiffBuilderLegacy;
+      assert.equal(builder._prefs.line_length, 50);
+    });
+
+    test(`line_length ignored for commit msg under ${mode}`, () => {
+      element.path = '/COMMIT_MSG';
+      element.viewMode = mode;
+      element.diff = createEmptyDiff();
+      element.prefs = {
+        ...createDefaultDiffPrefs(),
+        tab_size: 4,
+        line_length: 50,
+      };
+      builder = element.getDiffBuilder() as GrDiffBuilderLegacy;
+      assert.equal(builder._prefs.line_length, 72);
+    });
+  });
+
+  test('createTextEl linewrap with tabs', () => {
+    setBuilderPrefs({tab_size: 4, line_length: 10});
+    const text = '\t'.repeat(7) + '!';
+    const el = builder.createTextEl(null, line(text));
+    assert.equal(el.innerText, text);
+    // With line length 10 and tab size 4, there should be a line break
+    // after every two tabs.
+    const newlineEl = el.querySelector('.contentText > .br');
+    assert.isOk(newlineEl);
+    assert.equal(
+      el.querySelector('.contentText .tab:nth-child(2)')?.nextSibling,
+      newlineEl
+    );
+  });
+
+  test('_handlePreferenceError throws with invalid preference', () => {
+    element.prefs = {...createDefaultDiffPrefs(), tab_size: 0};
+    assert.throws(() => element.getDiffBuilder());
+  });
+
+  test('_handlePreferenceError triggers alert and javascript error', () => {
+    const errorStub = sinon.stub();
+    diffTable.addEventListener('show-alert', errorStub);
+    assert.throws(() => element.handlePreferenceError('tab size'));
+    assert.equal(
+      errorStub.lastCall.args[0].detail.message,
+      "The value of the 'tab size' user preference is invalid. " +
+        'Fix in diff preferences'
+    );
+  });
+
+  suite('intraline differences', () => {
+    let el: HTMLElement;
+    let str: string;
+    let annotateElementSpy: sinon.SinonSpy;
+    let layer: DiffLayer;
+    const lineNumberEl = document.createElement('td');
+
+    function slice(str: string, start: number, end?: number) {
+      return Array.from(str).slice(start, end).join('');
+    }
+
+    setup(async () => {
+      el = await fixture(html`
+        <div>Lorem ipsum dolor sit amet, suspendisse inceptos vehicula</div>
+      `);
+      str = el.textContent ?? '';
+      annotateElementSpy = sinon.spy(GrAnnotation, 'annotateElement');
+      layer = element.createIntralineLayer();
+    });
+
+    test('annotate no highlights', () => {
+      layer.annotate(el, lineNumberEl, line(str), Side.LEFT);
+
+      // The content is unchanged.
+      assert.isFalse(annotateElementSpy.called);
+      assert.equal(el.childNodes.length, 1);
+      assert.instanceOf(el.childNodes[0], Text);
+      assert.equal(str, el.childNodes[0].textContent);
+    });
+
+    test('annotate with highlights', () => {
+      const l = line(str);
+      l.highlights = [
+        {contentIndex: 0, startIndex: 6, endIndex: 12},
+        {contentIndex: 0, startIndex: 18, endIndex: 22},
+      ];
+      const str0 = slice(str, 0, 6);
+      const str1 = slice(str, 6, 12);
+      const str2 = slice(str, 12, 18);
+      const str3 = slice(str, 18, 22);
+      const str4 = slice(str, 22);
+
+      layer.annotate(el, lineNumberEl, l, Side.LEFT);
+
+      assert.isTrue(annotateElementSpy.called);
+      assert.equal(el.childNodes.length, 5);
+
+      assert.instanceOf(el.childNodes[0], Text);
+      assert.equal(el.childNodes[0].textContent, str0);
+
+      assert.notInstanceOf(el.childNodes[1], Text);
+      assert.equal(el.childNodes[1].textContent, str1);
+
+      assert.instanceOf(el.childNodes[2], Text);
+      assert.equal(el.childNodes[2].textContent, str2);
+
+      assert.notInstanceOf(el.childNodes[3], Text);
+      assert.equal(el.childNodes[3].textContent, str3);
+
+      assert.instanceOf(el.childNodes[4], Text);
+      assert.equal(el.childNodes[4].textContent, str4);
+    });
+
+    test('annotate without endIndex', () => {
+      const l = line(str);
+      l.highlights = [{contentIndex: 0, startIndex: 28}];
+
+      const str0 = slice(str, 0, 28);
+      const str1 = slice(str, 28);
+
+      layer.annotate(el, lineNumberEl, l, Side.LEFT);
+
+      assert.isTrue(annotateElementSpy.called);
+      assert.equal(el.childNodes.length, 2);
+
+      assert.instanceOf(el.childNodes[0], Text);
+      assert.equal(el.childNodes[0].textContent, str0);
+
+      assert.notInstanceOf(el.childNodes[1], Text);
+      assert.equal(el.childNodes[1].textContent, str1);
+    });
+
+    test('annotate ignores empty highlights', () => {
+      const l = line(str);
+      l.highlights = [{contentIndex: 0, startIndex: 28, endIndex: 28}];
+
+      layer.annotate(el, lineNumberEl, l, Side.LEFT);
+
+      assert.isFalse(annotateElementSpy.called);
+      assert.equal(el.childNodes.length, 1);
+    });
+
+    test('annotate handles unicode', () => {
+      // Put some unicode into the string:
+      str = str.replace(/\s/g, '💢');
+      el.textContent = str;
+      const l = line(str);
+      l.highlights = [{contentIndex: 0, startIndex: 6, endIndex: 12}];
+
+      const str0 = slice(str, 0, 6);
+      const str1 = slice(str, 6, 12);
+      const str2 = slice(str, 12);
+
+      layer.annotate(el, lineNumberEl, l, Side.LEFT);
+
+      assert.isTrue(annotateElementSpy.called);
+      assert.equal(el.childNodes.length, 3);
+
+      assert.instanceOf(el.childNodes[0], Text);
+      assert.equal(el.childNodes[0].textContent, str0);
+
+      assert.notInstanceOf(el.childNodes[1], Text);
+      assert.equal(el.childNodes[1].textContent, str1);
+
+      assert.instanceOf(el.childNodes[2], Text);
+      assert.equal(el.childNodes[2].textContent, str2);
+    });
+
+    test('annotate handles unicode w/o endIndex', () => {
+      // Put some unicode into the string:
+      str = str.replace(/\s/g, '💢');
+      el.textContent = str;
+
+      const l = line(str);
+      l.highlights = [{contentIndex: 0, startIndex: 6}];
+
+      const str0 = slice(str, 0, 6);
+      const str1 = slice(str, 6);
+
+      layer.annotate(el, lineNumberEl, l, Side.LEFT);
+
+      assert.isTrue(annotateElementSpy.called);
+      assert.equal(el.childNodes.length, 2);
+
+      assert.instanceOf(el.childNodes[0], Text);
+      assert.equal(el.childNodes[0].textContent, str0);
+
+      assert.notInstanceOf(el.childNodes[1], Text);
+      assert.equal(el.childNodes[1].textContent, str1);
+    });
+  });
+
+  suite('tab indicators', () => {
+    let layer: DiffLayer;
+    const lineNumberEl = document.createElement('td');
+
+    setup(() => {
+      element.showTabs = true;
+      layer = element.createTabIndicatorLayer();
+    });
+
+    test('does nothing with empty line', () => {
+      const l = line('');
+      const el = document.createElement('div');
+      const annotateElementStub = sinon.stub(GrAnnotation, 'annotateElement');
+
+      layer.annotate(el, lineNumberEl, l, Side.LEFT);
+
+      assert.isFalse(annotateElementStub.called);
+    });
+
+    test('does nothing with no tabs', () => {
+      const str = 'lorem ipsum no tabs';
+      const l = line(str);
+      const el = document.createElement('div');
+      el.textContent = str;
+      const annotateElementStub = sinon.stub(GrAnnotation, 'annotateElement');
+
+      layer.annotate(el, lineNumberEl, l, Side.LEFT);
+
+      assert.isFalse(annotateElementStub.called);
+    });
+
+    test('annotates tab at beginning', () => {
+      const str = '\tlorem upsum';
+      const l = line(str);
+      const el = document.createElement('div');
+      el.textContent = str;
+      const annotateElementStub = sinon.stub(GrAnnotation, 'annotateElement');
+
+      layer.annotate(el, lineNumberEl, l, Side.LEFT);
+
+      assert.equal(annotateElementStub.callCount, 1);
+      const args = annotateElementStub.getCalls()[0].args;
+      assert.equal(args[0], el);
+      assert.equal(args[1], 0, 'offset of tab indicator');
+      assert.equal(args[2], 1, 'length of tab indicator');
+      assert.include(args[3], 'tab-indicator');
+    });
+
+    test('does not annotate when disabled', () => {
+      element.showTabs = false;
+
+      const str = '\tlorem upsum';
+      const l = line(str);
+      const el = document.createElement('div');
+      el.textContent = str;
+      const annotateElementStub = sinon.stub(GrAnnotation, 'annotateElement');
+
+      layer.annotate(el, lineNumberEl, l, Side.LEFT);
+
+      assert.isFalse(annotateElementStub.called);
+    });
+
+    test('annotates multiple in beginning', () => {
+      const str = '\t\tlorem upsum';
+      const l = line(str);
+      const el = document.createElement('div');
+      el.textContent = str;
+      const annotateElementStub = sinon.stub(GrAnnotation, 'annotateElement');
+
+      layer.annotate(el, lineNumberEl, l, Side.LEFT);
+
+      assert.equal(annotateElementStub.callCount, 2);
+
+      let args = annotateElementStub.getCalls()[0].args;
+      assert.equal(args[0], el);
+      assert.equal(args[1], 0, 'offset of tab indicator');
+      assert.equal(args[2], 1, 'length of tab indicator');
+      assert.include(args[3], 'tab-indicator');
+
+      args = annotateElementStub.getCalls()[1].args;
+      assert.equal(args[0], el);
+      assert.equal(args[1], 1, 'offset of tab indicator');
+      assert.equal(args[2], 1, 'length of tab indicator');
+      assert.include(args[3], 'tab-indicator');
+    });
+
+    test('annotates intermediate tabs', () => {
+      const str = 'lorem\tupsum';
+      const l = line(str);
+      const el = document.createElement('div');
+      el.textContent = str;
+      const annotateElementStub = sinon.stub(GrAnnotation, 'annotateElement');
+
+      layer.annotate(el, lineNumberEl, l, Side.LEFT);
+
+      assert.equal(annotateElementStub.callCount, 1);
+      const args = annotateElementStub.getCalls()[0].args;
+      assert.equal(args[0], el);
+      assert.equal(args[1], 5, 'offset of tab indicator');
+      assert.equal(args[2], 1, 'length of tab indicator');
+      assert.include(args[3], 'tab-indicator');
+    });
+  });
+
+  suite('layers', () => {
+    let initialLayersCount = 0;
+    let withLayerCount = 0;
+    setup(() => {
+      const layers: DiffLayer[] = [];
+      element.layers = layers;
+      element.showTrailingWhitespace = true;
+      element.setupAnnotationLayers();
+      initialLayersCount = element.layersInternal.length;
+    });
+
+    test('no layers', () => {
+      element.setupAnnotationLayers();
+      assert.equal(element.layersInternal.length, initialLayersCount);
+    });
+
+    suite('with layers', () => {
+      const layers: DiffLayer[] = [{annotate: () => {}}, {annotate: () => {}}];
+      setup(() => {
+        element.layers = layers;
+        element.showTrailingWhitespace = true;
+        element.setupAnnotationLayers();
+        withLayerCount = element.layersInternal.length;
+      });
+      test('with layers', () => {
+        element.setupAnnotationLayers();
+        assert.equal(element.layersInternal.length, withLayerCount);
+        assert.equal(initialLayersCount + layers.length, withLayerCount);
+      });
+    });
+  });
+
+  suite('trailing whitespace', () => {
+    let layer: DiffLayer;
+    const lineNumberEl = document.createElement('td');
+
+    setup(() => {
+      element.showTrailingWhitespace = true;
+      layer = element.createTrailingWhitespaceLayer();
+    });
+
+    test('does nothing with empty line', () => {
+      const l = line('');
+      const el = document.createElement('div');
+      const annotateElementStub = sinon.stub(GrAnnotation, 'annotateElement');
+      layer.annotate(el, lineNumberEl, l, Side.LEFT);
+      assert.isFalse(annotateElementStub.called);
+    });
+
+    test('does nothing with no trailing whitespace', () => {
+      const str = 'lorem ipsum blah blah';
+      const l = line(str);
+      const el = document.createElement('div');
+      el.textContent = str;
+      const annotateElementStub = sinon.stub(GrAnnotation, 'annotateElement');
+      layer.annotate(el, lineNumberEl, l, Side.LEFT);
+      assert.isFalse(annotateElementStub.called);
+    });
+
+    test('annotates trailing spaces', () => {
+      const str = 'lorem ipsum   ';
+      const l = line(str);
+      const el = document.createElement('div');
+      el.textContent = str;
+      const annotateElementStub = sinon.stub(GrAnnotation, 'annotateElement');
+      layer.annotate(el, lineNumberEl, l, Side.LEFT);
+      assert.isTrue(annotateElementStub.called);
+      assert.equal(annotateElementStub.lastCall.args[1], 11);
+      assert.equal(annotateElementStub.lastCall.args[2], 3);
+    });
+
+    test('annotates trailing tabs', () => {
+      const str = 'lorem ipsum\t\t\t';
+      const l = line(str);
+      const el = document.createElement('div');
+      el.textContent = str;
+      const annotateElementStub = sinon.stub(GrAnnotation, 'annotateElement');
+      layer.annotate(el, lineNumberEl, l, Side.LEFT);
+      assert.isTrue(annotateElementStub.called);
+      assert.equal(annotateElementStub.lastCall.args[1], 11);
+      assert.equal(annotateElementStub.lastCall.args[2], 3);
+    });
+
+    test('annotates mixed trailing whitespace', () => {
+      const str = 'lorem ipsum\t \t';
+      const l = line(str);
+      const el = document.createElement('div');
+      el.textContent = str;
+      const annotateElementStub = sinon.stub(GrAnnotation, 'annotateElement');
+      layer.annotate(el, lineNumberEl, l, Side.LEFT);
+      assert.isTrue(annotateElementStub.called);
+      assert.equal(annotateElementStub.lastCall.args[1], 11);
+      assert.equal(annotateElementStub.lastCall.args[2], 3);
+    });
+
+    test('unicode preceding trailing whitespace', () => {
+      const str = '💢\t';
+      const l = line(str);
+      const el = document.createElement('div');
+      el.textContent = str;
+      const annotateElementStub = sinon.stub(GrAnnotation, 'annotateElement');
+      layer.annotate(el, lineNumberEl, l, Side.LEFT);
+      assert.isTrue(annotateElementStub.called);
+      assert.equal(annotateElementStub.lastCall.args[1], 1);
+      assert.equal(annotateElementStub.lastCall.args[2], 1);
+    });
+
+    test('does not annotate when disabled', () => {
+      element.showTrailingWhitespace = false;
+      const str = 'lorem upsum\t \t ';
+      const l = line(str);
+      const el = document.createElement('div');
+      el.textContent = str;
+      const annotateElementStub = sinon.stub(GrAnnotation, 'annotateElement');
+      layer.annotate(el, lineNumberEl, l, Side.LEFT);
+      assert.isFalse(annotateElementStub.called);
+    });
+  });
+
+  suite('rendering text, images and binary files', () => {
+    let processStub: sinon.SinonStub;
+    let keyLocations: KeyLocations;
+    let content: DiffContent[] = [];
+
+    setup(() => {
+      element.viewMode = 'SIDE_BY_SIDE';
+      processStub = sinon
+        .stub(element.processor, 'process')
+        .returns(Promise.resolve());
+      keyLocations = {left: {}, right: {}};
+      element.prefs = {
+        ...DEFAULT_PREFS,
+        context: -1,
+        syntax_highlighting: true,
+      };
+      content = [
+        {
+          a: ['all work and no play make andybons a dull boy'],
+          b: ['elgoog elgoog elgoog'],
+        },
+        {
+          ab: [
+            'Non eram nescius, Brute, cum, quae summis ingeniis ',
+            'exquisitaque doctrina philosophi Graeco sermone tractavissent',
+          ],
+        },
+      ];
+    });
+
+    test('text', async () => {
+      element.diff = {...createEmptyDiff(), content};
+      element.render(keyLocations);
+      await waitForEventOnce(diffTable, 'render-content');
+      assert.isTrue(processStub.calledOnce);
+      assert.isFalse(processStub.lastCall.args[1]);
+    });
+
+    test('image', async () => {
+      element.diff = {...createEmptyDiff(), content, binary: true};
+      element.isImageDiff = true;
+      element.render(keyLocations);
+      await waitForEventOnce(diffTable, 'render-content');
+      assert.isTrue(processStub.calledOnce);
+      assert.isTrue(processStub.lastCall.args[1]);
+    });
+
+    test('binary', async () => {
+      element.diff = {...createEmptyDiff(), content, binary: true};
+      element.render(keyLocations);
+      await waitForEventOnce(diffTable, 'render-content');
+      assert.isTrue(processStub.calledOnce);
+      assert.isTrue(processStub.lastCall.args[1]);
+    });
+  });
+
+  suite('rendering', () => {
+    let content: DiffContent[];
+    let outputEl: HTMLTableElement;
+    let keyLocations: KeyLocations;
+    let addColumnsStub: sinon.SinonStub;
+    let dispatchStub: sinon.SinonStub;
+    let builder: GrDiffBuilderSideBySide;
+
+    setup(() => {
+      const prefs = {...DEFAULT_PREFS};
+      content = [
+        {
+          a: ['all work and no play make andybons a dull boy'],
+          b: ['elgoog elgoog elgoog'],
+        },
+        {
+          ab: [
+            'Non eram nescius, Brute, cum, quae summis ingeniis ',
+            'exquisitaque doctrina philosophi Graeco sermone tractavissent',
+          ],
+        },
+      ];
+      dispatchStub = sinon.stub(diffTable, 'dispatchEvent');
+      outputEl = element.diffElement!;
+      keyLocations = {left: {}, right: {}};
+      sinon.stub(element, 'getDiffBuilder').callsFake(() => {
+        builder = new GrDiffBuilderSideBySide(
+          {...createEmptyDiff(), content},
+          prefs,
+          outputEl
+        );
+        addColumnsStub = sinon.stub(builder, 'addColumns');
+        builder.buildSectionElement = function (group) {
+          const section = document.createElement('stub');
+          section.textContent = group.lines.reduce(
+            (acc, line) => acc + line.text,
+            ''
+          );
+          return section;
+        };
+        return builder;
+      });
+      element.diff = {...createEmptyDiff(), content};
+      element.prefs = prefs;
+      element.render(keyLocations);
+    });
+
+    test('addColumns is called', () => {
+      assert.isTrue(addColumnsStub.called);
+    });
+
+    test('getGroupsByLineRange one line', () => {
+      const section = outputEl.querySelector<HTMLElement>(
+        'stub:nth-of-type(3)'
+      );
+      const groups = builder.getGroupsByLineRange(1, 1, Side.LEFT);
+      assert.equal(groups.length, 1);
+      assert.strictEqual(groups[0].element, section);
+    });
+
+    test('getGroupsByLineRange over diff', () => {
+      const section = [
+        outputEl.querySelector<HTMLElement>('stub:nth-of-type(3)'),
+        outputEl.querySelector<HTMLElement>('stub:nth-of-type(4)'),
+      ];
+      const groups = builder.getGroupsByLineRange(1, 2, Side.LEFT);
+      assert.equal(groups.length, 2);
+      assert.strictEqual(groups[0].element, section[0]);
+      assert.strictEqual(groups[1].element, section[1]);
+    });
+
+    test('render-start and render-content are fired', async () => {
+      await nextRender();
+      let firedEventTypes = dispatchStub.getCalls().map(c => c.args[0].type);
+      assert.include(firedEventTypes, 'render-start');
+      assert.include(firedEventTypes, 'render-progress');
+
+      await nextRender();
+      firedEventTypes = dispatchStub.getCalls().map(c => c.args[0].type);
+      assert.include(firedEventTypes, 'render-content');
+    });
+
+    test('cancel cancels the processor', () => {
+      const processorCancelStub = sinon.stub(element.processor, 'cancel');
+      element.cancel();
+      assert.isTrue(processorCancelStub.called);
+    });
+  });
+
+  suite('context hiding and expanding', () => {
+    let dispatchStub: sinon.SinonStub;
+
+    setup(async () => {
+      dispatchStub = sinon.stub(diffTable, 'dispatchEvent');
+      element.diff = {
+        ...createEmptyDiff(),
+        content: [
+          {ab: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10].map(i => `unchanged ${i}`)},
+          {a: ['before'], b: ['after']},
+          {ab: [1, 2, 3, 4, 5, 6, 7, 8, 9, 10].map(i => `unchanged ${10 + i}`)},
+        ],
+      };
+      element.viewMode = DiffViewMode.SIDE_BY_SIDE;
+
+      const keyLocations: KeyLocations = {left: {}, right: {}};
+      element.prefs = {
+        ...DEFAULT_PREFS,
+        context: 1,
+      };
+      element.render(keyLocations);
+      // Make sure all listeners are installed.
+      await nextRender();
+    });
+
+    test('hides lines behind two context controls', () => {
+      const contextControls = diffTable.querySelectorAll('gr-context-controls');
+      assert.equal(contextControls.length, 2);
+
+      const diffRows = diffTable.querySelectorAll('.diff-row');
+      // The first two are LOST and FILE line
+      assert.equal(diffRows.length, 2 + 1 + 1 + 1);
+      assert.include(diffRows[2].textContent, 'unchanged 10');
+      assert.include(diffRows[3].textContent, 'before');
+      assert.include(diffRows[3].textContent, 'after');
+      assert.include(diffRows[4].textContent, 'unchanged 11');
+    });
+
+    test('clicking +x common lines expands those lines', () => {
+      const contextControls = diffTable.querySelectorAll('gr-context-controls');
+      const topExpandCommonButton =
+        contextControls[0].shadowRoot?.querySelectorAll<HTMLElement>(
+          '.showContext'
+        )[0];
+      assert.isOk(topExpandCommonButton);
+      assert.include(topExpandCommonButton!.textContent, '+9 common lines');
+      topExpandCommonButton!.click();
+      const diffRows = diffTable.querySelectorAll('.diff-row');
+      // The first two are LOST and FILE line
+      assert.equal(diffRows.length, 2 + 10 + 1 + 1);
+      assert.include(diffRows[2].textContent, 'unchanged 1');
+      assert.include(diffRows[3].textContent, 'unchanged 2');
+      assert.include(diffRows[4].textContent, 'unchanged 3');
+      assert.include(diffRows[5].textContent, 'unchanged 4');
+      assert.include(diffRows[6].textContent, 'unchanged 5');
+      assert.include(diffRows[7].textContent, 'unchanged 6');
+      assert.include(diffRows[8].textContent, 'unchanged 7');
+      assert.include(diffRows[9].textContent, 'unchanged 8');
+      assert.include(diffRows[10].textContent, 'unchanged 9');
+      assert.include(diffRows[11].textContent, 'unchanged 10');
+      assert.include(diffRows[12].textContent, 'before');
+      assert.include(diffRows[12].textContent, 'after');
+      assert.include(diffRows[13].textContent, 'unchanged 11');
+    });
+
+    test('unhideLine shows the line with context', async () => {
+      dispatchStub.reset();
+      element.unhideLine(4, Side.LEFT);
+
+      const diffRows = diffTable.querySelectorAll('.diff-row');
+      // The first two are LOST and FILE line
+      // Lines 3-5 (Line 4 plus 1 context in each direction) will be expanded
+      // Because context expanders do not hide <3 lines, lines 1-2 will also
+      // be shown.
+      // Lines 6-9 continue to be hidden
+      assert.equal(diffRows.length, 2 + 5 + 1 + 1 + 1);
+      assert.include(diffRows[2].textContent, 'unchanged 1');
+      assert.include(diffRows[3].textContent, 'unchanged 2');
+      assert.include(diffRows[4].textContent, 'unchanged 3');
+      assert.include(diffRows[5].textContent, 'unchanged 4');
+      assert.include(diffRows[6].textContent, 'unchanged 5');
+      assert.include(diffRows[7].textContent, 'unchanged 10');
+      assert.include(diffRows[8].textContent, 'before');
+      assert.include(diffRows[8].textContent, 'after');
+      assert.include(diffRows[9].textContent, 'unchanged 11');
+
+      await nextRender();
+      const firedEventTypes = dispatchStub.getCalls().map(c => c.args[0].type);
+      assert.include(firedEventTypes, 'render-content');
+    });
+  });
+
+  suite('mock-diff', () => {
+    let builder: GrDiffBuilderSideBySide;
+    let diff: DiffInfo;
+    let keyLocations: KeyLocations;
+
+    setup(() => {
+      element.viewMode = DiffViewMode.SIDE_BY_SIDE;
+      diff = createDiff();
+      element.diff = diff;
+
+      keyLocations = {left: {}, right: {}};
+
+      element.prefs = {
+        ...createDefaultDiffPrefs(),
+        line_length: 80,
+        show_tabs: true,
+        tab_size: 4,
+      };
+      element.render(keyLocations);
+      builder = element.builder as GrDiffBuilderSideBySide;
+    });
+
+    test('aria-labels on added line numbers', () => {
+      const deltaLineNumberButton = diffTable.querySelectorAll(
+        '.lineNumButton.right'
+      )[5];
+
+      assert.isOk(deltaLineNumberButton);
+      assert.equal(deltaLineNumberButton.getAttribute('aria-label'), '5 added');
+    });
+
+    test('aria-labels on removed line numbers', () => {
+      const deltaLineNumberButton = diffTable.querySelectorAll(
+        '.lineNumButton.left'
+      )[10];
+
+      assert.isOk(deltaLineNumberButton);
+      assert.equal(
+        deltaLineNumberButton.getAttribute('aria-label'),
+        '10 removed'
+      );
+    });
+
+    test('getContentByLine', () => {
+      let actual: HTMLElement | null;
+
+      actual = builder.getContentByLine(2, Side.LEFT);
+      assert.equal(actual?.textContent, diff.content[0].ab?.[1]);
+
+      actual = builder.getContentByLine(2, Side.RIGHT);
+      assert.equal(actual?.textContent, diff.content[0].ab?.[1]);
+
+      actual = builder.getContentByLine(5, Side.LEFT);
+      assert.equal(actual?.textContent, diff.content[2].ab?.[0]);
+
+      actual = builder.getContentByLine(5, Side.RIGHT);
+      assert.equal(actual?.textContent, diff.content[1].b?.[0]);
+    });
+
+    test('getContentTdByLineEl works both with button and td', () => {
+      const diffRow = diffTable.querySelectorAll('tr.diff-row')[2];
+
+      const lineNumTdLeft = queryAndAssert(diffRow, 'td.lineNum.left');
+      const lineNumButtonLeft = queryAndAssert(lineNumTdLeft, 'button');
+      const contentTdLeft = diffRow.querySelectorAll('.content')[0];
+
+      const lineNumTdRight = queryAndAssert(diffRow, 'td.lineNum.right');
+      const lineNumButtonRight = queryAndAssert(lineNumTdRight, 'button');
+      const contentTdRight = diffRow.querySelectorAll('.content')[1];
+
+      assert.equal(element.getContentTdByLineEl(lineNumTdLeft), contentTdLeft);
+      assert.equal(
+        element.getContentTdByLineEl(lineNumButtonLeft),
+        contentTdLeft
+      );
+      assert.equal(
+        element.getContentTdByLineEl(lineNumTdRight),
+        contentTdRight
+      );
+      assert.equal(
+        element.getContentTdByLineEl(lineNumButtonRight),
+        contentTdRight
+      );
+    });
+
+    test('findLinesByRange', () => {
+      const lines: GrDiffLine[] = [];
+      const elems: HTMLElement[] = [];
+      const start = 6;
+      const end = 10;
+      const count = end - start + 1;
+
+      builder.findLinesByRange(start, end, Side.RIGHT, lines, elems);
+
+      assert.equal(lines.length, count);
+      assert.equal(elems.length, count);
+
+      for (let i = 0; i < 5; i++) {
+        assert.instanceOf(lines[i], GrDiffLine);
+        assert.equal(lines[i].afterNumber, start + i);
+        assert.instanceOf(elems[i], HTMLElement);
+        assert.equal(lines[i].text, elems[i].textContent);
+      }
+    });
+
+    test('renderContentByRange', () => {
+      const spy = sinon.spy(builder, 'createTextEl');
+      const start = 9;
+      const end = 14;
+      const count = end - start + 1;
+
+      builder.renderContentByRange(start, end, Side.LEFT);
+
+      assert.equal(spy.callCount, count);
+      spy.getCalls().forEach((call, i: number) => {
+        assert.equal(call.args[1].beforeNumber, start + i);
+      });
+    });
+
+    test('renderContentByRange non-existent elements', () => {
+      const spy = sinon.spy(builder, 'createTextEl');
+
+      sinon
+        .stub(builder, 'getLineNumberEl')
+        .returns(document.createElement('div'));
+      sinon
+        .stub(builder, 'findLinesByRange')
+        .callsFake((_1, _2, _3, lines, elements) => {
+          // Add a line and a corresponding element.
+          lines?.push(new GrDiffLine(GrDiffLineType.BOTH));
+          const tr = document.createElement('tr');
+          const td = document.createElement('td');
+          const el = document.createElement('div');
+          tr.appendChild(td);
+          td.appendChild(el);
+          elements?.push(el);
+
+          // Add 2 lines without corresponding elements.
+          lines?.push(new GrDiffLine(GrDiffLineType.BOTH));
+          lines?.push(new GrDiffLine(GrDiffLineType.BOTH));
+        });
+
+      builder.renderContentByRange(1, 10, Side.LEFT);
+      // Should be called only once because only one line had a corresponding
+      // element.
+      assert.equal(spy.callCount, 1);
+    });
+
+    test('getLineNumberEl side-by-side left', () => {
+      const contentEl = builder.getContentByLine(
+        5,
+        Side.LEFT,
+        element.diffElement as HTMLTableElement
+      );
+      assert.isOk(contentEl);
+      const lineNumberEl = builder.getLineNumberEl(contentEl!, Side.LEFT);
+      assert.isOk(lineNumberEl);
+      assert.isTrue(lineNumberEl!.classList.contains('lineNum'));
+      assert.isTrue(lineNumberEl!.classList.contains(Side.LEFT));
+    });
+
+    test('getLineNumberEl side-by-side right', () => {
+      const contentEl = builder.getContentByLine(
+        5,
+        Side.RIGHT,
+        element.diffElement as HTMLTableElement
+      );
+      assert.isOk(contentEl);
+      const lineNumberEl = builder.getLineNumberEl(contentEl!, Side.RIGHT);
+      assert.isOk(lineNumberEl);
+      assert.isTrue(lineNumberEl!.classList.contains('lineNum'));
+      assert.isTrue(lineNumberEl!.classList.contains(Side.RIGHT));
+    });
+
+    test('getLineNumberEl unified left', async () => {
+      // Re-render as unified:
+      element.viewMode = 'UNIFIED_DIFF';
+      element.render(keyLocations);
+      builder = element.builder as GrDiffBuilderSideBySide;
+
+      const contentEl = builder.getContentByLine(
+        5,
+        Side.LEFT,
+        element.diffElement as HTMLTableElement
+      );
+      assert.isOk(contentEl);
+      const lineNumberEl = builder.getLineNumberEl(contentEl!, Side.LEFT);
+      assert.isOk(lineNumberEl);
+      assert.isTrue(lineNumberEl!.classList.contains('lineNum'));
+      assert.isTrue(lineNumberEl!.classList.contains(Side.LEFT));
+    });
+
+    test('getLineNumberEl unified right', async () => {
+      // Re-render as unified:
+      element.viewMode = 'UNIFIED_DIFF';
+      element.render(keyLocations);
+      builder = element.builder as GrDiffBuilderSideBySide;
+
+      const contentEl = builder.getContentByLine(
+        5,
+        Side.RIGHT,
+        element.diffElement as HTMLTableElement
+      );
+      assert.isOk(contentEl);
+      const lineNumberEl = builder.getLineNumberEl(contentEl!, Side.RIGHT);
+      assert.isOk(lineNumberEl);
+      assert.isTrue(lineNumberEl!.classList.contains('lineNum'));
+      assert.isTrue(lineNumberEl!.classList.contains(Side.RIGHT));
+    });
+
+    test('getNextContentOnSide side-by-side left', () => {
+      const startElem = builder.getContentByLine(
+        5,
+        Side.LEFT,
+        element.diffElement as HTMLTableElement
+      );
+      assert.isOk(startElem);
+      const expectedStartString = diff.content[2].ab?.[0];
+      const expectedNextString = diff.content[2].ab?.[1];
+      assert.equal(startElem!.textContent, expectedStartString);
+
+      const nextElem = builder.getNextContentOnSide(startElem!, Side.LEFT);
+      assert.isOk(nextElem);
+      assert.equal(nextElem!.textContent, expectedNextString);
+    });
+
+    test('getNextContentOnSide side-by-side right', () => {
+      const startElem = builder.getContentByLine(
+        5,
+        Side.RIGHT,
+        element.diffElement as HTMLTableElement
+      );
+      const expectedStartString = diff.content[1].b?.[0];
+      const expectedNextString = diff.content[1].b?.[1];
+      assert.isOk(startElem);
+      assert.equal(startElem!.textContent, expectedStartString);
+
+      const nextElem = builder.getNextContentOnSide(startElem!, Side.RIGHT);
+      assert.isOk(nextElem);
+      assert.equal(nextElem!.textContent, expectedNextString);
+    });
+
+    test('getNextContentOnSide unified left', async () => {
+      // Re-render as unified:
+      element.viewMode = 'UNIFIED_DIFF';
+      element.render(keyLocations);
+      builder = element.builder as GrDiffBuilderSideBySide;
+
+      const startElem = builder.getContentByLine(
+        5,
+        Side.LEFT,
+        element.diffElement as HTMLTableElement
+      );
+      const expectedStartString = diff.content[2].ab?.[0];
+      const expectedNextString = diff.content[2].ab?.[1];
+      assert.isOk(startElem);
+      assert.equal(startElem!.textContent, expectedStartString);
+
+      const nextElem = builder.getNextContentOnSide(startElem!, Side.LEFT);
+      assert.isOk(nextElem);
+      assert.equal(nextElem!.textContent, expectedNextString);
+    });
+
+    test('getNextContentOnSide unified right', async () => {
+      // Re-render as unified:
+      element.viewMode = 'UNIFIED_DIFF';
+      element.render(keyLocations);
+      builder = element.builder as GrDiffBuilderSideBySide;
+
+      const startElem = builder.getContentByLine(
+        5,
+        Side.RIGHT,
+        element.diffElement as HTMLTableElement
+      );
+      const expectedStartString = diff.content[1].b?.[0];
+      const expectedNextString = diff.content[1].b?.[1];
+      assert.isOk(startElem);
+      assert.equal(startElem!.textContent, expectedStartString);
+
+      const nextElem = builder.getNextContentOnSide(startElem!, Side.RIGHT);
+      assert.isOk(nextElem);
+      assert.equal(nextElem!.textContent, expectedNextString);
+    });
+  });
+
+  suite('blame', () => {
+    let mockBlame: BlameInfo[];
+
+    setup(() => {
+      mockBlame = [
+        {
+          author: 'test-author',
+          time: 314,
+          commit_msg: 'test-commit-message',
+          id: 'commit 1',
+          ranges: [
+            {start: 1, end: 2},
+            {start: 10, end: 16},
+          ],
+        },
+        {
+          author: 'test-author',
+          time: 314,
+          commit_msg: 'test-commit-message',
+          id: 'commit 2',
+          ranges: [
+            {start: 4, end: 10},
+            {start: 17, end: 32},
+          ],
+        },
+      ];
+    });
+
+    test('setBlame attempts to render each blamed line', () => {
+      const getBlameStub = sinon
+        .stub(builder, 'getBlameTdByLine')
+        .returns(undefined);
+      builder.setBlame(mockBlame);
+      assert.equal(getBlameStub.callCount, 32);
+    });
+
+    test('getBlameCommitForBaseLine', () => {
+      sinon.stub(builder, 'getBlameTdByLine').returns(undefined);
+      builder.setBlame(mockBlame);
+      assert.isOk(builder.getBlameCommitForBaseLine(1));
+      assert.equal(builder.getBlameCommitForBaseLine(1)?.id, 'commit 1');
+
+      assert.isOk(builder.getBlameCommitForBaseLine(11));
+      assert.equal(builder.getBlameCommitForBaseLine(11)?.id, 'commit 1');
+
+      assert.isOk(builder.getBlameCommitForBaseLine(32));
+      assert.equal(builder.getBlameCommitForBaseLine(32)?.id, 'commit 2');
+
+      assert.isUndefined(builder.getBlameCommitForBaseLine(33));
+    });
+
+    test('getBlameCommitForBaseLine w/o blame returns null', () => {
+      assert.isUndefined(builder.getBlameCommitForBaseLine(1));
+      assert.isUndefined(builder.getBlameCommitForBaseLine(11));
+      assert.isUndefined(builder.getBlameCommitForBaseLine(31));
+    });
+
+    test('createBlameCell', () => {
+      const mockBlameInfo = {
+        time: 1576155200,
+        id: '1234567890',
+        author: 'Clark Kent',
+        commit_msg: 'Testing Commit',
+        ranges: [{start: 4, end: 10}],
+      };
+      const getBlameStub = sinon
+        .stub(builder, 'getBlameCommitForBaseLine')
+        .returns(mockBlameInfo);
+      const line = new GrDiffLine(GrDiffLineType.BOTH);
+      line.beforeNumber = 3;
+      line.afterNumber = 5;
+
+      const result = builder.createBlameCell(line.beforeNumber);
+
+      assert.isTrue(getBlameStub.calledWithExactly(3));
+      assert.equal(result.getAttribute('data-line-number'), '3');
+      expect(result).dom.to.equal(/* HTML */ `
+        <span class="gr-diff style-scope">
+          <a class="blameDate gr-diff style-scope" href="/r/q/1234567890">
+            12/12/2019
+          </a>
+          <span class="blameAuthor gr-diff style-scope">Clark</span>
+          <gr-hovercard class="gr-diff style-scope">
+            <span class="blameHoverCard gr-diff style-scope">
+              Commit 1234567890<br />
+              Author: Clark Kent<br />
+              Date: 12/12/2019<br />
+              <br />
+              Testing Commit
+            </span>
+          </gr-hovercard>
+        </span>
+      `);
+    });
+  });
+});
diff --git a/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-legacy.ts b/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-legacy.ts
index ceadc94..c04d156 100644
--- a/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-legacy.ts
+++ b/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-legacy.ts
@@ -162,10 +162,8 @@
    *
    * TODO(brohlfs): Consolidate this with getLineEl... methods in html file.
    */
-  private getLineNumberEl(
-    content: HTMLElement,
-    side: Side
-  ): HTMLElement | null {
+  // visible for testing
+  getLineNumberEl(content: HTMLElement, side: Side): HTMLElement | null {
     let row: HTMLElement | null = content;
     while (row && !row.classList.contains('diff-row')) row = row.parentElement;
     return row ? (row.querySelector('.lineNum.' + side) as HTMLElement) : null;
@@ -349,7 +347,8 @@
     });
   }
 
-  protected createTextEl(
+  // visible for testing
+  createTextEl(
     lineNumberEl: HTMLElement | null,
     line: GrDiffLine,
     side?: Side
@@ -491,7 +490,8 @@
    * Create a blame cell for the given base line. Blame information will be
    * included in the cell if available.
    */
-  protected createBlameCell(lineNumber: LineNumber): HTMLTableCellElement {
+  // visible for testing
+  createBlameCell(lineNumber: LineNumber): HTMLTableCellElement {
     const blameTd = createElementDiff('td', 'blame') as HTMLTableCellElement;
     blameTd.setAttribute('data-line-number', lineNumber.toString());
     if (!lineNumber) return blameTd;
diff --git a/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-side-by-side.ts b/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-side-by-side.ts
index a711215..f2690bc 100644
--- a/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-side-by-side.ts
+++ b/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-side-by-side.ts
@@ -44,7 +44,8 @@
     };
   }
 
-  protected override buildSectionElement(group: GrDiffGroup) {
+  // visible for testing
+  override buildSectionElement(group: GrDiffGroup) {
     const sectionEl = createElementDiff('tbody', 'section');
     sectionEl.classList.add(group.type);
     if (group.isTotal()) {
@@ -147,7 +148,8 @@
     return td;
   }
 
-  protected override getNextContentOnSide(
+  // visible for testing
+  override getNextContentOnSide(
     content: HTMLElement,
     side: Side
   ): HTMLElement | null {
diff --git a/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-unified.ts b/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-unified.ts
index 4145485..0c9d1d9 100644
--- a/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-unified.ts
+++ b/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-unified.ts
@@ -43,7 +43,8 @@
     };
   }
 
-  protected override buildSectionElement(group: GrDiffGroup): HTMLElement {
+  // visible for testing
+  override buildSectionElement(group: GrDiffGroup): HTMLElement {
     const sectionEl = createElementDiff('tbody', 'section');
     sectionEl.classList.add(group.type);
     if (group.isTotal()) {
diff --git a/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-unified_test.js b/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-unified_test.js
deleted file mode 100644
index 5f3fb72..0000000
--- a/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-unified_test.js
+++ /dev/null
@@ -1,242 +0,0 @@
-/**
- * @license
- * Copyright (C) 2016 The Android Open Source Project
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- * http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-import '../../../test/common-test-setup-karma.js';
-import '../gr-diff/gr-diff-group.js';
-import './gr-diff-builder.js';
-import './gr-diff-builder-unified.js';
-import {GrDiffLine, GrDiffLineType} from '../gr-diff/gr-diff-line.js';
-import {GrDiffGroup, GrDiffGroupType} from '../gr-diff/gr-diff-group.js';
-import {GrDiffBuilderUnified} from './gr-diff-builder-unified.js';
-
-suite('GrDiffBuilderUnified tests', () => {
-  let prefs;
-  let outputEl;
-  let diffBuilder;
-
-  setup(()=> {
-    prefs = {
-      line_length: 10,
-      show_tabs: true,
-      tab_size: 4,
-    };
-    outputEl = document.createElement('div');
-    diffBuilder = new GrDiffBuilderUnified({}, prefs, outputEl, []);
-  });
-
-  suite('buildSectionElement for BOTH group', () => {
-    let lines;
-    let group;
-
-    setup(() => {
-      lines = [
-        new GrDiffLine(GrDiffLineType.BOTH, 1, 2),
-        new GrDiffLine(GrDiffLineType.BOTH, 2, 3),
-        new GrDiffLine(GrDiffLineType.BOTH, 3, 4),
-      ];
-      lines[0].text = 'def hello_world():';
-      lines[1].text = '  print "Hello World";';
-      lines[2].text = '  return True';
-
-      group = new GrDiffGroup({type: GrDiffGroupType.BOTH, lines});
-    });
-
-    test('creates the section', () => {
-      const sectionEl = diffBuilder.buildSectionElement(group);
-      assert.isTrue(sectionEl.classList.contains('section'));
-      assert.isTrue(sectionEl.classList.contains('both'));
-    });
-
-    test('creates each unchanged row once', () => {
-      const sectionEl = diffBuilder.buildSectionElement(group);
-      const rowEls = sectionEl.querySelectorAll('.diff-row');
-
-      assert.equal(rowEls.length, 3);
-
-      assert.equal(
-          rowEls[0].querySelector('.lineNum.left').textContent,
-          lines[0].beforeNumber);
-      assert.equal(
-          rowEls[0].querySelector('.lineNum.right').textContent,
-          lines[0].afterNumber);
-      assert.equal(
-          rowEls[0].querySelector('.content').textContent, lines[0].text);
-
-      assert.equal(
-          rowEls[1].querySelector('.lineNum.left').textContent,
-          lines[1].beforeNumber);
-      assert.equal(
-          rowEls[1].querySelector('.lineNum.right').textContent,
-          lines[1].afterNumber);
-      assert.equal(
-          rowEls[1].querySelector('.content').textContent, lines[1].text);
-
-      assert.equal(
-          rowEls[2].querySelector('.lineNum.left').textContent,
-          lines[2].beforeNumber);
-      assert.equal(
-          rowEls[2].querySelector('.lineNum.right').textContent,
-          lines[2].afterNumber);
-      assert.equal(
-          rowEls[2].querySelector('.content').textContent, lines[2].text);
-    });
-  });
-
-  suite('buildSectionElement for moved chunks', () => {
-    test('creates a moved out group', () => {
-      const lines = [
-        new GrDiffLine(GrDiffLineType.REMOVE, 15),
-        new GrDiffLine(GrDiffLineType.REMOVE, 16),
-      ];
-      lines[0].text = 'def hello_world():';
-      lines[1].text = '  print "Hello World"';
-      const group = new GrDiffGroup({type: GrDiffGroupType.DELTA, lines});
-      group.moveDetails = {changed: false};
-
-      const sectionEl = diffBuilder.buildSectionElement(group);
-
-      const rowEls = sectionEl.querySelectorAll('tr');
-      const moveControlsRow = rowEls[0];
-      const cells = moveControlsRow.querySelectorAll('td');
-      assert.isTrue(sectionEl.classList.contains('dueToMove'));
-      assert.equal(rowEls.length, 3);
-      assert.isTrue(moveControlsRow.classList.contains('movedOut'));
-      assert.equal(cells.length, 3);
-      assert.isTrue(cells[2].classList.contains('moveHeader'));
-      assert.equal(cells[2].textContent, 'Moved out');
-    });
-
-    test('creates a moved in group', () => {
-      const lines = [
-        new GrDiffLine(GrDiffLineType.ADD, 37),
-        new GrDiffLine(GrDiffLineType.ADD, 38),
-      ];
-      lines[0].text = 'def hello_world():';
-      lines[1].text = '  print "Hello World"';
-      const group = new GrDiffGroup({type: GrDiffGroupType.DELTA, lines});
-      group.moveDetails = {changed: false};
-
-      const sectionEl = diffBuilder.buildSectionElement(group);
-
-      const rowEls = sectionEl.querySelectorAll('tr');
-      const moveControlsRow = rowEls[0];
-      const cells = moveControlsRow.querySelectorAll('td');
-      assert.isTrue(sectionEl.classList.contains('dueToMove'));
-      assert.equal(rowEls.length, 3);
-      assert.isTrue(moveControlsRow.classList.contains('movedIn'));
-      assert.equal(cells.length, 3);
-      assert.isTrue(cells[2].classList.contains('moveHeader'));
-      assert.equal(cells[2].textContent, 'Moved in');
-    });
-  });
-
-  suite('buildSectionElement for DELTA group', () => {
-    let lines;
-    let group;
-
-    setup(() => {
-      lines = [
-        new GrDiffLine(GrDiffLineType.REMOVE, 1),
-        new GrDiffLine(GrDiffLineType.REMOVE, 2),
-        new GrDiffLine(GrDiffLineType.ADD, 2),
-        new GrDiffLine(GrDiffLineType.ADD, 3),
-      ];
-      lines[0].text = 'def hello_world():';
-      lines[1].text = '  print "Hello World"';
-      lines[2].text = 'def hello_universe()';
-      lines[3].text = '  print "Hello Universe"';
-
-      group = new GrDiffGroup({type: GrDiffGroupType.DELTA, lines});
-    });
-
-    test('creates the section', () => {
-      const sectionEl = diffBuilder.buildSectionElement(group);
-      assert.isTrue(sectionEl.classList.contains('section'));
-      assert.isTrue(sectionEl.classList.contains('delta'));
-    });
-
-    test('creates the section with class if ignoredWhitespaceOnly', () => {
-      group.ignoredWhitespaceOnly = true;
-      const sectionEl = diffBuilder.buildSectionElement(group);
-      assert.isTrue(sectionEl.classList.contains('ignoredWhitespaceOnly'));
-    });
-
-    test('creates the section with class if dueToRebase', () => {
-      group.dueToRebase = true;
-      const sectionEl = diffBuilder.buildSectionElement(group);
-      assert.isTrue(sectionEl.classList.contains('dueToRebase'));
-    });
-
-    test('creates first the removed and then the added rows', () => {
-      const sectionEl = diffBuilder.buildSectionElement(group);
-      const rowEls = sectionEl.querySelectorAll('.diff-row');
-
-      assert.equal(rowEls.length, 4);
-
-      assert.equal(
-          rowEls[0].querySelector('.lineNum.left').textContent,
-          lines[0].beforeNumber);
-      assert.isNotOk(rowEls[0].querySelector('.lineNum.right'));
-      assert.equal(
-          rowEls[0].querySelector('.content').textContent, lines[0].text);
-
-      assert.equal(
-          rowEls[1].querySelector('.lineNum.left').textContent,
-          lines[1].beforeNumber);
-      assert.isNotOk(rowEls[1].querySelector('.lineNum.right'));
-      assert.equal(
-          rowEls[1].querySelector('.content').textContent, lines[1].text);
-
-      assert.isNotOk(rowEls[2].querySelector('.lineNum.left'));
-      assert.equal(
-          rowEls[2].querySelector('.lineNum.right').textContent,
-          lines[2].afterNumber);
-      assert.equal(
-          rowEls[2].querySelector('.content').textContent, lines[2].text);
-
-      assert.isNotOk(rowEls[3].querySelector('.lineNum.left'));
-      assert.equal(
-          rowEls[3].querySelector('.lineNum.right').textContent,
-          lines[3].afterNumber);
-      assert.equal(
-          rowEls[3].querySelector('.content').textContent, lines[3].text);
-    });
-
-    test('creates only the added rows if only ignored whitespace', () => {
-      group.ignoredWhitespaceOnly = true;
-      const sectionEl = diffBuilder.buildSectionElement(group);
-      const rowEls = sectionEl.querySelectorAll('.diff-row');
-
-      assert.equal(rowEls.length, 2);
-
-      assert.isNotOk(rowEls[0].querySelector('.lineNum.left'));
-      assert.equal(
-          rowEls[0].querySelector('.lineNum.right').textContent,
-          lines[2].afterNumber);
-      assert.equal(
-          rowEls[0].querySelector('.content').textContent, lines[2].text);
-
-      assert.isNotOk(rowEls[1].querySelector('.lineNum.left'));
-      assert.equal(
-          rowEls[1].querySelector('.lineNum.right').textContent,
-          lines[3].afterNumber);
-      assert.equal(
-          rowEls[1].querySelector('.content').textContent, lines[3].text);
-    });
-  });
-});
-
diff --git a/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-unified_test.ts b/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-unified_test.ts
new file mode 100644
index 0000000..7a9d06d
--- /dev/null
+++ b/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-unified_test.ts
@@ -0,0 +1,282 @@
+/**
+ * @license
+ * Copyright 2016 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+import '../../../test/common-test-setup-karma';
+import '../gr-diff/gr-diff-group';
+import './gr-diff-builder';
+import './gr-diff-builder-unified';
+import {GrDiffLine, GrDiffLineType} from '../gr-diff/gr-diff-line';
+import {GrDiffGroup, GrDiffGroupType} from '../gr-diff/gr-diff-group';
+import {GrDiffBuilderUnified} from './gr-diff-builder-unified';
+import {DiffPreferencesInfo} from '../../../api/diff';
+import {createDefaultDiffPrefs} from '../../../constants/constants';
+import {createDiff} from '../../../test/test-data-generators';
+import {queryAndAssert} from '../../../utils/common-util';
+
+suite('GrDiffBuilderUnified tests', () => {
+  let prefs: DiffPreferencesInfo;
+  let outputEl: HTMLElement;
+  let diffBuilder: GrDiffBuilderUnified;
+
+  setup(() => {
+    prefs = {
+      ...createDefaultDiffPrefs(),
+      line_length: 10,
+      show_tabs: true,
+      tab_size: 4,
+    };
+    outputEl = document.createElement('div');
+    diffBuilder = new GrDiffBuilderUnified(createDiff(), prefs, outputEl, []);
+  });
+
+  suite('buildSectionElement for BOTH group', () => {
+    let lines: GrDiffLine[];
+    let group: GrDiffGroup;
+
+    setup(() => {
+      lines = [
+        new GrDiffLine(GrDiffLineType.BOTH, 1, 2),
+        new GrDiffLine(GrDiffLineType.BOTH, 2, 3),
+        new GrDiffLine(GrDiffLineType.BOTH, 3, 4),
+      ];
+      lines[0].text = 'def hello_world():';
+      lines[1].text = '  print "Hello World";';
+      lines[2].text = '  return True';
+
+      group = new GrDiffGroup({type: GrDiffGroupType.BOTH, lines});
+    });
+
+    test('creates the section', () => {
+      const sectionEl = diffBuilder.buildSectionElement(group);
+      assert.isTrue(sectionEl.classList.contains('section'));
+      assert.isTrue(sectionEl.classList.contains('both'));
+    });
+
+    test('creates each unchanged row once', () => {
+      const sectionEl = diffBuilder.buildSectionElement(group);
+      const rowEls = sectionEl.querySelectorAll('.diff-row');
+
+      assert.equal(rowEls.length, 3);
+
+      assert.equal(
+        queryAndAssert(rowEls[0], '.lineNum.left').textContent,
+        lines[0].beforeNumber.toString()
+      );
+      assert.equal(
+        queryAndAssert(rowEls[0], '.lineNum.right').textContent,
+        lines[0].afterNumber.toString()
+      );
+      assert.equal(
+        queryAndAssert(rowEls[0], '.content').textContent,
+        lines[0].text
+      );
+
+      assert.equal(
+        queryAndAssert(rowEls[1], '.lineNum.left').textContent,
+        lines[1].beforeNumber.toString()
+      );
+      assert.equal(
+        queryAndAssert(rowEls[1], '.lineNum.right').textContent,
+        lines[1].afterNumber.toString()
+      );
+      assert.equal(
+        queryAndAssert(rowEls[1], '.content').textContent,
+        lines[1].text
+      );
+
+      assert.equal(
+        queryAndAssert(rowEls[2], '.lineNum.left').textContent,
+        lines[2].beforeNumber.toString()
+      );
+      assert.equal(
+        queryAndAssert(rowEls[2], '.lineNum.right').textContent,
+        lines[2].afterNumber.toString()
+      );
+      assert.equal(
+        queryAndAssert(rowEls[2], '.content').textContent,
+        lines[2].text
+      );
+    });
+  });
+
+  suite('buildSectionElement for moved chunks', () => {
+    test('creates a moved out group', () => {
+      const lines = [
+        new GrDiffLine(GrDiffLineType.REMOVE, 15),
+        new GrDiffLine(GrDiffLineType.REMOVE, 16),
+      ];
+      lines[0].text = 'def hello_world():';
+      lines[1].text = '  print "Hello World"';
+      const group = new GrDiffGroup({
+        type: GrDiffGroupType.DELTA,
+        lines,
+        moveDetails: {changed: false},
+      });
+
+      const sectionEl = diffBuilder.buildSectionElement(group);
+
+      const rowEls = sectionEl.querySelectorAll('tr');
+      const moveControlsRow = rowEls[0];
+      const cells = moveControlsRow.querySelectorAll('td');
+      assert.isTrue(sectionEl.classList.contains('dueToMove'));
+      assert.equal(rowEls.length, 3);
+      assert.isTrue(moveControlsRow.classList.contains('movedOut'));
+      assert.equal(cells.length, 3);
+      assert.isTrue(cells[2].classList.contains('moveHeader'));
+      assert.equal(cells[2].textContent, 'Moved out');
+    });
+
+    test('creates a moved in group', () => {
+      const lines = [
+        new GrDiffLine(GrDiffLineType.ADD, 37),
+        new GrDiffLine(GrDiffLineType.ADD, 38),
+      ];
+      lines[0].text = 'def hello_world():';
+      lines[1].text = '  print "Hello World"';
+      const group = new GrDiffGroup({
+        type: GrDiffGroupType.DELTA,
+        lines,
+        moveDetails: {changed: false},
+      });
+
+      const sectionEl = diffBuilder.buildSectionElement(group);
+
+      const rowEls = sectionEl.querySelectorAll('tr');
+      const moveControlsRow = rowEls[0];
+      const cells = moveControlsRow.querySelectorAll('td');
+      assert.isTrue(sectionEl.classList.contains('dueToMove'));
+      assert.equal(rowEls.length, 3);
+      assert.isTrue(moveControlsRow.classList.contains('movedIn'));
+      assert.equal(cells.length, 3);
+      assert.isTrue(cells[2].classList.contains('moveHeader'));
+      assert.equal(cells[2].textContent, 'Moved in');
+    });
+  });
+
+  suite('buildSectionElement for DELTA group', () => {
+    let lines: GrDiffLine[];
+    let group: GrDiffGroup;
+
+    setup(() => {
+      lines = [
+        new GrDiffLine(GrDiffLineType.REMOVE, 1),
+        new GrDiffLine(GrDiffLineType.REMOVE, 2),
+        new GrDiffLine(GrDiffLineType.ADD, 2),
+        new GrDiffLine(GrDiffLineType.ADD, 3),
+      ];
+      lines[0].text = 'def hello_world():';
+      lines[1].text = '  print "Hello World"';
+      lines[2].text = 'def hello_universe()';
+      lines[3].text = '  print "Hello Universe"';
+    });
+
+    test('creates the section', () => {
+      group = new GrDiffGroup({type: GrDiffGroupType.DELTA, lines});
+      const sectionEl = diffBuilder.buildSectionElement(group);
+      assert.isTrue(sectionEl.classList.contains('section'));
+      assert.isTrue(sectionEl.classList.contains('delta'));
+    });
+
+    test('creates the section with class if ignoredWhitespaceOnly', () => {
+      group = new GrDiffGroup({
+        type: GrDiffGroupType.DELTA,
+        lines,
+        ignoredWhitespaceOnly: true,
+      });
+      const sectionEl = diffBuilder.buildSectionElement(group);
+      assert.isTrue(sectionEl.classList.contains('ignoredWhitespaceOnly'));
+    });
+
+    test('creates the section with class if dueToRebase', () => {
+      group = new GrDiffGroup({
+        type: GrDiffGroupType.DELTA,
+        lines,
+        dueToRebase: true,
+      });
+      const sectionEl = diffBuilder.buildSectionElement(group);
+      assert.isTrue(sectionEl.classList.contains('dueToRebase'));
+    });
+
+    test('creates first the removed and then the added rows', () => {
+      group = new GrDiffGroup({type: GrDiffGroupType.DELTA, lines});
+      const sectionEl = diffBuilder.buildSectionElement(group);
+      const rowEls = sectionEl.querySelectorAll('.diff-row');
+
+      assert.equal(rowEls.length, 4);
+
+      assert.equal(
+        queryAndAssert(rowEls[0], '.lineNum.left').textContent,
+        lines[0].beforeNumber.toString()
+      );
+      assert.isNotOk(rowEls[0].querySelector('.lineNum.right'));
+      assert.equal(
+        queryAndAssert(rowEls[0], '.content').textContent,
+        lines[0].text
+      );
+
+      assert.equal(
+        queryAndAssert(rowEls[1], '.lineNum.left').textContent,
+        lines[1].beforeNumber.toString()
+      );
+      assert.isNotOk(rowEls[1].querySelector('.lineNum.right'));
+      assert.equal(
+        queryAndAssert(rowEls[1], '.content').textContent,
+        lines[1].text
+      );
+
+      assert.isNotOk(rowEls[2].querySelector('.lineNum.left'));
+      assert.equal(
+        queryAndAssert(rowEls[2], '.lineNum.right').textContent,
+        lines[2].afterNumber.toString()
+      );
+      assert.equal(
+        queryAndAssert(rowEls[2], '.content').textContent,
+        lines[2].text
+      );
+
+      assert.isNotOk(rowEls[3].querySelector('.lineNum.left'));
+      assert.equal(
+        queryAndAssert(rowEls[3], '.lineNum.right').textContent,
+        lines[3].afterNumber.toString()
+      );
+      assert.equal(
+        queryAndAssert(rowEls[3], '.content').textContent,
+        lines[3].text
+      );
+    });
+
+    test('creates only the added rows if only ignored whitespace', () => {
+      group = new GrDiffGroup({
+        type: GrDiffGroupType.DELTA,
+        lines,
+        ignoredWhitespaceOnly: true,
+      });
+      const sectionEl = diffBuilder.buildSectionElement(group);
+      const rowEls = sectionEl.querySelectorAll('.diff-row');
+
+      assert.equal(rowEls.length, 2);
+
+      assert.isNotOk(rowEls[0].querySelector('.lineNum.left'));
+      assert.equal(
+        queryAndAssert(rowEls[0], '.lineNum.right').textContent,
+        lines[2].afterNumber.toString()
+      );
+      assert.equal(
+        queryAndAssert(rowEls[0], '.content').textContent,
+        lines[2].text
+      );
+
+      assert.isNotOk(rowEls[1].querySelector('.lineNum.left'));
+      assert.equal(
+        queryAndAssert(rowEls[1], '.lineNum.right').textContent,
+        lines[3].afterNumber.toString()
+      );
+      assert.equal(
+        queryAndAssert(rowEls[1], '.content').textContent,
+        lines[3].text
+      );
+    });
+  });
+});
diff --git a/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder.ts b/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder.ts
index add7ffa..4b664e2 100644
--- a/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder.ts
+++ b/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder.ts
@@ -89,7 +89,8 @@
 
   protected readonly numLinesLeft: number;
 
-  protected readonly _prefs: DiffPreferencesInfo;
+  // visible for testing
+  readonly _prefs: DiffPreferencesInfo;
 
   protected readonly renderPrefs?: RenderPreferences;
 
@@ -194,7 +195,8 @@
     group.element = element;
   }
 
-  private getGroupsByLineRange(
+  // visible for testing
+  getGroupsByLineRange(
     startLine: LineNumber,
     endLine: LineNumber,
     side: Side
@@ -257,7 +259,8 @@
    *        TODO: Change `null` to `undefined` in paramete type. Also: Do we
    *        really need to support null/undefined? Also change to camelCase.
    */
-  protected findLinesByRange(
+  // visible for testing
+  findLinesByRange(
     start: LineNumber,
     end: LineNumber,
     side: Side,
@@ -352,9 +355,8 @@
    *
    * @return The commit information.
    */
-  protected getBlameCommitForBaseLine(
-    lineNum: LineNumber
-  ): BlameInfo | undefined {
+  // visible for testing
+  getBlameCommitForBaseLine(lineNum: LineNumber): BlameInfo | undefined {
     for (const blameCommit of this.blameInfo) {
       for (const range of blameCommit.ranges) {
         if (range.start <= lineNum && range.end >= lineNum) {
diff --git a/polygerrit-ui/app/embed/diff/gr-diff-cursor/gr-diff-cursor.ts b/polygerrit-ui/app/embed/diff/gr-diff-cursor/gr-diff-cursor.ts
index dfe8a15..e80d86b 100644
--- a/polygerrit-ui/app/embed/diff/gr-diff-cursor/gr-diff-cursor.ts
+++ b/polygerrit-ui/app/embed/diff/gr-diff-cursor/gr-diff-cursor.ts
@@ -109,7 +109,8 @@
    */
   initialLineNumber: number | null = null;
 
-  private cursorManager = new GrCursorManager();
+  // visible for testing
+  cursorManager = new GrCursorManager();
 
   private targetSubscription?: Subscription;
 
diff --git a/polygerrit-ui/app/embed/diff/gr-diff-cursor/gr-diff-cursor_test.js b/polygerrit-ui/app/embed/diff/gr-diff-cursor/gr-diff-cursor_test.js
deleted file mode 100644
index f48d673..0000000
--- a/polygerrit-ui/app/embed/diff/gr-diff-cursor/gr-diff-cursor_test.js
+++ /dev/null
@@ -1,681 +0,0 @@
-/**
- * @license
- * Copyright (C) 2016 The Android Open Source Project
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- * http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-import '../../../test/common-test-setup-karma.js';
-import '../gr-diff/gr-diff.js';
-import './gr-diff-cursor.js';
-import {fixture, html} from '@open-wc/testing-helpers';
-import {listenOnce, mockPromise} from '../../../test/test-utils.js';
-import {createDiff} from '../../../test/test-data-generators.js';
-import {createDefaultDiffPrefs} from '../../../constants/constants.js';
-import {GrDiffCursor} from './gr-diff-cursor.js';
-import {afterNextRender} from '@polymer/polymer/lib/utils/render-status';
-
-suite('gr-diff-cursor tests', () => {
-  let cursor;
-  let diffElement;
-  let diff;
-
-  setup(async () => {
-    diffElement = await fixture(html`<gr-diff></gr-diff>`);
-    cursor = new GrDiffCursor();
-
-    // Register the diff with the cursor.
-    cursor.replaceDiffs([diffElement]);
-
-    diffElement.loggedIn = false;
-    diffElement.comments = {
-      left: [],
-      right: [],
-      meta: {},
-    };
-    diffElement.path = 'some/path.ts';
-    const promise = mockPromise();
-    const setupDone = () => {
-      cursor._updateStops();
-      cursor.moveToFirstChunk();
-      diffElement.removeEventListener('render', setupDone);
-      promise.resolve();
-    };
-    diffElement.addEventListener('render', setupDone);
-
-    diff = createDiff();
-    diffElement.prefs = createDefaultDiffPrefs();
-    diffElement.diff = diff;
-    await promise;
-  });
-
-  test('diff cursor functionality (side-by-side)', () => {
-    // The cursor has been initialized to the first delta.
-    assert.isOk(cursor.diffRow);
-
-    const firstDeltaRow = diffElement.shadowRoot
-        .querySelector('.section.delta .diff-row');
-    assert.equal(cursor.diffRow, firstDeltaRow);
-
-    cursor.moveDown();
-
-    assert.notEqual(cursor.diffRow, firstDeltaRow);
-    assert.equal(cursor.diffRow, firstDeltaRow.nextSibling);
-
-    cursor.moveUp();
-
-    assert.notEqual(cursor.diffRow, firstDeltaRow.nextSibling);
-    assert.equal(cursor.diffRow, firstDeltaRow);
-  });
-
-  test('moveToFirstChunk', async () => {
-    const diff = {
-      meta_a: {
-        name: 'lorem-ipsum.txt',
-        content_type: 'text/plain',
-        lines: 3,
-      },
-      meta_b: {
-        name: 'lorem-ipsum.txt',
-        content_type: 'text/plain',
-        lines: 3,
-      },
-      intraline_status: 'OK',
-      change_type: 'MODIFIED',
-      diff_header: [
-        'diff --git a/lorem-ipsum.txt b/lorem-ipsum.txt',
-        'index b2adcf4..554ae49 100644',
-        '--- a/lorem-ipsum.txt',
-        '+++ b/lorem-ipsum.txt',
-      ],
-      content: [
-        {b: ['new line 1']},
-        {ab: ['unchanged line']},
-        {a: ['old line 2']},
-        {ab: ['more unchanged lines']},
-      ],
-    };
-
-    diffElement.diff = diff;
-    // The file comment button, if present, is a cursor stop. Ensure
-    // moveToFirstChunk() works correctly even if the button is not shown.
-    diffElement.prefs.show_file_comment_button = false;
-    await flush();
-    cursor._updateStops();
-
-    const chunks = Array.from(diffElement.root.querySelectorAll(
-        '.section.delta'));
-    assert.equal(chunks.length, 2);
-
-    // Verify it works on fresh diff.
-    cursor.moveToFirstChunk();
-    assert.equal(chunks.indexOf(cursor.diffRow.parentElement), 0);
-    assert.equal(cursor.side, 'right');
-
-    // Verify it works from other cursor positions.
-    cursor.moveToNextChunk();
-    assert.equal(chunks.indexOf(cursor.diffRow.parentElement), 1);
-    assert.equal(cursor.side, 'left');
-    cursor.moveToFirstChunk();
-    assert.equal(chunks.indexOf(cursor.diffRow.parentElement), 0);
-    assert.equal(cursor.side, 'right');
-  });
-
-  test('moveToLastChunk', async () => {
-    const diff = {
-      meta_a: {
-        name: 'lorem-ipsum.txt',
-        content_type: 'text/plain',
-        lines: 3,
-      },
-      meta_b: {
-        name: 'lorem-ipsum.txt',
-        content_type: 'text/plain',
-        lines: 3,
-      },
-      intraline_status: 'OK',
-      change_type: 'MODIFIED',
-      diff_header: [
-        'diff --git a/lorem-ipsum.txt b/lorem-ipsum.txt',
-        'index b2adcf4..554ae49 100644',
-        '--- a/lorem-ipsum.txt',
-        '+++ b/lorem-ipsum.txt',
-      ],
-      content: [
-        {ab: ['unchanged line']},
-        {a: ['old line 2']},
-        {ab: ['more unchanged lines']},
-        {b: ['new line 3']},
-      ],
-    };
-
-    diffElement.diff = diff;
-    await new Promise(resolve => afterNextRender(diffElement, resolve));
-    cursor._updateStops();
-
-    const chunks = Array.from(diffElement.root.querySelectorAll(
-        '.section.delta'));
-    assert.equal(chunks.length, 2);
-
-    // Verify it works on fresh diff.
-    cursor.moveToLastChunk();
-    assert.equal(chunks.indexOf(cursor.diffRow.parentElement), 1);
-    assert.equal(cursor.side, 'right');
-
-    // Verify it works from other cursor positions.
-    cursor.moveToPreviousChunk();
-    assert.equal(chunks.indexOf(cursor.diffRow.parentElement), 0);
-    assert.equal(cursor.side, 'left');
-    cursor.moveToLastChunk();
-    assert.equal(chunks.indexOf(cursor.diffRow.parentElement), 1);
-    assert.equal(cursor.side, 'right');
-  });
-
-  test('cursor scroll behavior', () => {
-    assert.equal(cursor.cursorManager.scrollMode, 'keep-visible');
-
-    diffElement.dispatchEvent(new Event('render-start'));
-    assert.isTrue(cursor.cursorManager.focusOnMove);
-
-    window.dispatchEvent(new Event('scroll'));
-    assert.equal(cursor.cursorManager.scrollMode, 'never');
-    assert.isFalse(cursor.cursorManager.focusOnMove);
-
-    diffElement.dispatchEvent(new Event('render-content'));
-    assert.isTrue(cursor.cursorManager.focusOnMove);
-
-    cursor.reInitCursor();
-    assert.equal(cursor.cursorManager.scrollMode, 'keep-visible');
-  });
-
-  test('moves to selected line', () => {
-    const moveToNumStub = sinon.stub(cursor, 'moveToLineNumber');
-
-    diffElement.dispatchEvent(
-        new CustomEvent('line-selected', {
-          detail: {number: '123', side: 'right', path: 'some/file'},
-        }));
-
-    assert.isTrue(moveToNumStub.called);
-    assert.equal(moveToNumStub.lastCall.args[0], '123');
-    assert.equal(moveToNumStub.lastCall.args[1], 'right');
-    assert.equal(moveToNumStub.lastCall.args[2], 'some/file');
-  });
-
-  suite('unified diff', () => {
-    setup(async () => {
-      diffElement.viewMode = 'UNIFIED_DIFF';
-      // We must allow the diff to re-render after setting the viewMode.
-      await new Promise(resolve => afterNextRender(diffElement, resolve));
-      cursor.reInitCursor();
-    });
-
-    test('diff cursor functionality (unified)', () => {
-      // The cursor has been initialized to the first delta.
-      assert.isOk(cursor.diffRow);
-
-      let firstDeltaRow = diffElement.shadowRoot
-          .querySelector('.section.delta .diff-row');
-      assert.equal(cursor.diffRow, firstDeltaRow);
-
-      firstDeltaRow = diffElement.shadowRoot
-          .querySelector('.section.delta .diff-row');
-      assert.equal(cursor.diffRow, firstDeltaRow);
-
-      cursor.moveDown();
-
-      assert.notEqual(cursor.diffRow, firstDeltaRow);
-      assert.equal(cursor.diffRow, firstDeltaRow.nextSibling);
-
-      cursor.moveUp();
-
-      assert.notEqual(cursor.diffRow, firstDeltaRow.nextSibling);
-      assert.equal(cursor.diffRow, firstDeltaRow);
-    });
-  });
-
-  test('cursor side functionality', () => {
-    // The side only applies to side-by-side mode, which should be the default
-    // mode.
-    assert.equal(diffElement.viewMode, 'SIDE_BY_SIDE');
-
-    const firstDeltaSection = diffElement.shadowRoot
-        .querySelector('.section.delta');
-    const firstDeltaRow = firstDeltaSection.querySelector('.diff-row');
-
-    // Because the first delta in this diff is on the right, it should be set
-    // to the right side.
-    assert.equal(cursor.side, 'right');
-    assert.equal(cursor.diffRow, firstDeltaRow);
-    const firstIndex = cursor.cursorManager.index;
-
-    // Move the side to the left. Because this delta only has a right side, we
-    // should be moved up to the previous line where there is content on the
-    // right. The previous row is part of the previous section.
-    cursor.moveLeft();
-
-    assert.equal(cursor.side, 'left');
-    assert.notEqual(cursor.diffRow, firstDeltaRow);
-    assert.equal(cursor.cursorManager.index, firstIndex - 1);
-    assert.equal(cursor.diffRow.parentElement,
-        firstDeltaSection.previousSibling);
-
-    // If we move down, we should skip everything in the first delta because
-    // we are on the left side and the first delta has no content on the left.
-    cursor.moveDown();
-
-    assert.equal(cursor.side, 'left');
-    assert.notEqual(cursor.diffRow, firstDeltaRow);
-    assert.isTrue(cursor.cursorManager.index > firstIndex);
-    assert.equal(cursor.diffRow.parentElement,
-        firstDeltaSection.nextSibling);
-  });
-
-  test('chunk skip functionality', () => {
-    const chunks = diffElement.root.querySelectorAll(
-        '.section.delta');
-    const indexOfChunk = function(chunk) {
-      return Array.prototype.indexOf.call(chunks, chunk);
-    };
-
-    // We should be initialized to the first chunk. Since this chunk only has
-    // content on the right side, our side should be right.
-    let currentIndex = indexOfChunk(cursor.diffRow.parentElement);
-    assert.equal(currentIndex, 0);
-    assert.equal(cursor.side, 'right');
-
-    // Move to the next chunk.
-    cursor.moveToNextChunk();
-
-    // Since this chunk only has content on the left side. we should have been
-    // automatically moved over.
-    const previousIndex = currentIndex;
-    currentIndex = indexOfChunk(cursor.diffRow.parentElement);
-    assert.equal(currentIndex, previousIndex + 1);
-    assert.equal(cursor.side, 'left');
-  });
-
-  suite('moved chunks without line range)', () => {
-    setup(async () => {
-      const promise = mockPromise();
-      const renderHandler = function() {
-        diffElement.removeEventListener('render', renderHandler);
-        cursor.reInitCursor();
-        promise.resolve();
-      };
-      diffElement.addEventListener('render', renderHandler);
-      diffElement.diff = {...diff, content: [
-        {
-          ab: [
-            'Lorem ipsum dolor sit amet, suspendisse inceptos vehicula, ',
-          ],
-        },
-        {
-          b: [
-            'Nullam neque, ligula ac, id blandit.',
-            'Sagittis tincidunt torquent, tempor nunc amet.',
-            'At rhoncus id.',
-          ],
-          move_details: {changed: false},
-        },
-        {
-          ab: [
-            'Sem nascetur, erat ut, non in.',
-          ],
-        },
-        {
-          a: [
-            'Nullam neque, ligula ac, id blandit.',
-            'Sagittis tincidunt torquent, tempor nunc amet.',
-            'At rhoncus id.',
-          ],
-          move_details: {changed: false},
-        },
-        {
-          ab: [
-            'Arcu eget, rhoncus amet cursus, ipsum elementum.',
-          ],
-        },
-      ]};
-      await promise;
-    });
-
-    test('renders moveControls with simple descriptions', () => {
-      const [movedIn, movedOut] = diffElement.root
-          .querySelectorAll('.dueToMove .moveControls');
-      assert.equal(movedIn.textContent, 'Moved in');
-      assert.equal(movedOut.textContent, 'Moved out');
-    });
-  });
-
-  suite('moved chunks (moveDetails)', () => {
-    setup(async () => {
-      const promise = mockPromise();
-      const renderHandler = function() {
-        diffElement.removeEventListener('render', renderHandler);
-        cursor.reInitCursor();
-        promise.resolve();
-      };
-      diffElement.addEventListener('render', renderHandler);
-      diffElement.diff = {...diff, content: [
-        {
-          ab: [
-            'Lorem ipsum dolor sit amet, suspendisse inceptos vehicula, ',
-          ],
-        },
-        {
-          b: [
-            'Nullam neque, ligula ac, id blandit.',
-            'Sagittis tincidunt torquent, tempor nunc amet.',
-            'At rhoncus id.',
-          ],
-          move_details: {changed: false, range: {start: 4, end: 6}},
-        },
-        {
-          ab: [
-            'Sem nascetur, erat ut, non in.',
-          ],
-        },
-        {
-          a: [
-            'Nullam neque, ligula ac, id blandit.',
-            'Sagittis tincidunt torquent, tempor nunc amet.',
-            'At rhoncus id.',
-          ],
-          move_details: {changed: false, range: {start: 2, end: 4}},
-        },
-        {
-          ab: [
-            'Arcu eget, rhoncus amet cursus, ipsum elementum.',
-          ],
-        },
-      ]};
-      await promise;
-    });
-
-    test('renders moveControls with simple descriptions', () => {
-      const [movedIn, movedOut] = diffElement.root
-          .querySelectorAll('.dueToMove .moveControls');
-      assert.equal(movedIn.textContent, 'Moved from lines 4 - 6');
-      assert.equal(movedOut.textContent, 'Moved to lines 2 - 4');
-    });
-
-    test('startLineAnchor of movedIn chunk fires events', async () => {
-      const [movedIn] = diffElement.root
-          .querySelectorAll('.dueToMove .moveControls');
-      const [startLineAnchor] = movedIn.querySelectorAll('a');
-
-      const promise = mockPromise();
-      const onMovedLinkClicked = e => {
-        assert.deepEqual(e.detail, {lineNum: 4, side: 'left'});
-        promise.resolve();
-      };
-      assert.equal(startLineAnchor.textContent, '4');
-      startLineAnchor
-          .addEventListener('moved-link-clicked', onMovedLinkClicked);
-      MockInteractions.click(startLineAnchor);
-      await promise;
-    });
-
-    test('endLineAnchor of movedOut fires events', async () => {
-      const [, movedOut] = diffElement.root
-          .querySelectorAll('.dueToMove .moveControls');
-      const [, endLineAnchor] = movedOut.querySelectorAll('a');
-
-      const promise = mockPromise();
-      const onMovedLinkClicked = e => {
-        assert.deepEqual(e.detail, {lineNum: 4, side: 'right'});
-        promise.resolve();
-      };
-      assert.equal(endLineAnchor.textContent, '4');
-      endLineAnchor.addEventListener('moved-link-clicked', onMovedLinkClicked);
-      MockInteractions.click(endLineAnchor);
-      await promise;
-    });
-  });
-
-  test('initialLineNumber not provided', async () => {
-    let scrollBehaviorDuringMove;
-    const moveToNumStub = sinon.stub(cursor, 'moveToLineNumber');
-    const moveToChunkStub = sinon.stub(cursor, 'moveToFirstChunk')
-        .callsFake(() => {
-          scrollBehaviorDuringMove = cursor.cursorManager.scrollMode;
-        });
-
-    diffElement._diffChanged(createDiff());
-    await new Promise(resolve => afterNextRender(diffElement, resolve));
-    cursor.reInitCursor();
-    assert.isFalse(moveToNumStub.called);
-    assert.isTrue(moveToChunkStub.called);
-    assert.equal(scrollBehaviorDuringMove, 'never');
-    assert.equal(cursor.cursorManager.scrollMode, 'keep-visible');
-  });
-
-  test('initialLineNumber provided', async () => {
-    let scrollBehaviorDuringMove;
-    const moveToNumStub = sinon.stub(cursor, 'moveToLineNumber')
-        .callsFake(() => {
-          scrollBehaviorDuringMove = cursor.cursorManager.scrollMode;
-        });
-    const moveToChunkStub = sinon.stub(cursor, 'moveToFirstChunk');
-    cursor.initialLineNumber = 10;
-    cursor.side = 'right';
-
-    diffElement._diffChanged(createDiff());
-    await new Promise(resolve => afterNextRender(diffElement, resolve));
-    cursor.reInitCursor();
-    assert.isFalse(moveToChunkStub.called);
-    assert.isTrue(moveToNumStub.called);
-    assert.equal(moveToNumStub.lastCall.args[0], 10);
-    assert.equal(moveToNumStub.lastCall.args[1], 'right');
-    assert.equal(scrollBehaviorDuringMove, 'keep-visible');
-    assert.equal(cursor.cursorManager.scrollMode, 'keep-visible');
-  });
-
-  test('getTargetDiffElement', () => {
-    cursor.initialLineNumber = 1;
-    assert.isTrue(!!cursor.diffRow);
-    assert.equal(
-        cursor.getTargetDiffElement(),
-        diffElement
-    );
-  });
-
-  suite('createCommentInPlace', () => {
-    setup(() => {
-      diffElement.loggedIn = true;
-    });
-
-    test('adds new draft for selected line on the left', async () => {
-      cursor.moveToLineNumber(2, 'left');
-      const promise = mockPromise();
-      diffElement.addEventListener('create-comment', e => {
-        const {lineNum, range, side} = e.detail;
-        assert.equal(lineNum, 2);
-        assert.equal(range, undefined);
-        assert.equal(side, 'left');
-        promise.resolve();
-      });
-      cursor.createCommentInPlace();
-      await promise;
-    });
-
-    test('adds draft for selected line on the right', async () => {
-      cursor.moveToLineNumber(4, 'right');
-      const promise = mockPromise();
-      diffElement.addEventListener('create-comment', e => {
-        const {lineNum, range, side} = e.detail;
-        assert.equal(lineNum, 4);
-        assert.equal(range, undefined);
-        assert.equal(side, 'right');
-        promise.resolve();
-      });
-      cursor.createCommentInPlace();
-      await promise;
-    });
-
-    test('creates comment for range if selected', async () => {
-      const someRange = {
-        start_line: 2,
-        start_character: 3,
-        end_line: 6,
-        end_character: 1,
-      };
-      diffElement.highlights.selectedRange = {
-        side: 'right',
-        range: someRange,
-      };
-      const promise = mockPromise();
-      diffElement.addEventListener('create-comment', e => {
-        const {lineNum, range, side} = e.detail;
-        assert.equal(lineNum, 6);
-        assert.equal(range, someRange);
-        assert.equal(side, 'right');
-        promise.resolve();
-      });
-      cursor.createCommentInPlace();
-      await promise;
-    });
-
-    test('ignores call if nothing is selected', () => {
-      const createRangeCommentStub = sinon.stub(diffElement,
-          'createRangeComment');
-      const addDraftAtLineStub = sinon.stub(diffElement, 'addDraftAtLine');
-      cursor.diffRow = undefined;
-      cursor.createCommentInPlace();
-      assert.isFalse(createRangeCommentStub.called);
-      assert.isFalse(addDraftAtLineStub.called);
-    });
-  });
-
-  test('getAddress', () => {
-    // It should initialize to the first chunk: line 5 of the revision.
-    assert.deepEqual(cursor.getAddress(),
-        {leftSide: false, number: 5});
-
-    // Revision line 4 is up.
-    cursor.moveUp();
-    assert.deepEqual(cursor.getAddress(),
-        {leftSide: false, number: 4});
-
-    // Base line 4 is left.
-    cursor.moveLeft();
-    assert.deepEqual(cursor.getAddress(), {leftSide: true, number: 4});
-
-    // Moving to the next chunk takes it back to the start.
-    cursor.moveToNextChunk();
-    assert.deepEqual(cursor.getAddress(),
-        {leftSide: false, number: 5});
-
-    // The following chunk is a removal starting on line 10 of the base.
-    cursor.moveToNextChunk();
-    assert.deepEqual(cursor.getAddress(),
-        {leftSide: true, number: 10});
-
-    // Should be null if there is no selection.
-    cursor.cursorManager.unsetCursor();
-    assert.isNotOk(cursor.getAddress());
-  });
-
-  test('_findRowByNumberAndFile', () => {
-    // Get the first ab row after the first chunk.
-    const row = diffElement.root.querySelectorAll('tr')[9];
-
-    // It should be line 8 on the right, but line 5 on the left.
-    assert.equal(cursor._findRowByNumberAndFile(8, 'right'), row);
-    assert.equal(cursor._findRowByNumberAndFile(5, 'left'), row);
-  });
-
-  test('expand context updates stops', async () => {
-    sinon.spy(cursor, '_updateStops');
-    MockInteractions.tap(diffElement.shadowRoot
-        .querySelector('gr-context-controls').shadowRoot
-        .querySelector('.showContext'));
-    await new Promise(resolve => afterNextRender(diffElement, resolve));
-    assert.isTrue(cursor._updateStops.called);
-  });
-
-  test('updates stops when loading changes', () => {
-    sinon.spy(cursor, '_updateStops');
-    diffElement.dispatchEvent(new Event('loading-changed'));
-    assert.isTrue(cursor._updateStops.called);
-  });
-
-  suite('multi diff', () => {
-    let diffElements;
-
-    setup(async () => {
-      diffElements = [
-        await fixture(html`<gr-diff></gr-diff>`),
-        await fixture(html`<gr-diff></gr-diff>`),
-        await fixture(html`<gr-diff></gr-diff>`),
-      ];
-      cursor = new GrDiffCursor();
-
-      // Register the diff with the cursor.
-      cursor.replaceDiffs(diffElements);
-
-      for (const el of diffElements) {
-        el.prefs = createDefaultDiffPrefs();
-      }
-    });
-
-    function getTargetDiffIndex() {
-      // Mocha has a bug where when `assert.equals` fails, it will try to
-      // JSON.stringify the operands, which fails when they are cyclic structures
-      // like GrDiffElement. The failure is difficult to attribute to a specific
-      // assertion because of the async nature assertion errors are handled and
-      // can cause the test simply timing out, causing a lot of debugging headache.
-      // Working with indices circumvents the problem.
-      return diffElements.indexOf(cursor.getTargetDiffElement());
-    }
-
-    test('do not skip loading diffs', async () => {
-      const diffRenderedPromises =
-          diffElements.map(diffEl => listenOnce(diffEl, 'render'));
-
-      diffElements[0].diff = createDiff();
-      diffElements[2].diff = createDiff();
-      await Promise.all([diffRenderedPromises[0], diffRenderedPromises[2]]);
-      await new Promise(resolve => afterNextRender(diffElements[0], resolve));
-
-      const lastLine = diffElements[0].diff.meta_b.lines;
-
-      // Goto second last line of the first diff
-      cursor.moveToLineNumber(lastLine - 1, 'right');
-      assert.equal(
-          cursor.getTargetLineElement().textContent, lastLine - 1);
-
-      // Can move down until we reach the loading file
-      cursor.moveDown();
-      assert.equal(getTargetDiffIndex(), 0);
-      assert.equal(cursor.getTargetLineElement().textContent, lastLine);
-
-      // Cannot move down while still loading the diff we would switch to
-      cursor.moveDown();
-      assert.equal(getTargetDiffIndex(), 0);
-      assert.equal(cursor.getTargetLineElement().textContent, lastLine);
-
-      // Diff 1 finishing to load
-      diffElements[1].diff = createDiff();
-      await diffRenderedPromises[1];
-      await new Promise(resolve => afterNextRender(diffElements[0], resolve));
-
-      // Now we can go down
-      cursor.moveDown();
-      assert.equal(getTargetDiffIndex(), 1);
-      assert.equal(cursor.getTargetLineElement().textContent, 'File');
-    });
-  });
-});
-
diff --git a/polygerrit-ui/app/embed/diff/gr-diff-cursor/gr-diff-cursor_test.ts b/polygerrit-ui/app/embed/diff/gr-diff-cursor/gr-diff-cursor_test.ts
new file mode 100644
index 0000000..ac9b407
--- /dev/null
+++ b/polygerrit-ui/app/embed/diff/gr-diff-cursor/gr-diff-cursor_test.ts
@@ -0,0 +1,693 @@
+/**
+ * @license
+ * Copyright 2016 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+import '../../../test/common-test-setup-karma';
+import '../gr-diff/gr-diff';
+import './gr-diff-cursor';
+import {fixture, html} from '@open-wc/testing-helpers';
+import {mockPromise, queryAll, queryAndAssert} from '../../../test/test-utils';
+import {createDiff} from '../../../test/test-data-generators';
+import {createDefaultDiffPrefs} from '../../../constants/constants';
+import {GrDiffCursor} from './gr-diff-cursor';
+import {waitForEventOnce} from '../../../utils/event-util';
+import {DiffInfo, DiffViewMode, Side} from '../../../api/diff';
+import {GrDiff} from '../gr-diff/gr-diff';
+import {assertIsDefined} from '../../../utils/common-util';
+
+suite('gr-diff-cursor tests', () => {
+  let cursor: GrDiffCursor;
+  let diffElement: GrDiff;
+  let diff: DiffInfo;
+
+  setup(async () => {
+    diffElement = await fixture(html`<gr-diff></gr-diff>`);
+    cursor = new GrDiffCursor();
+
+    // Register the diff with the cursor.
+    cursor.replaceDiffs([diffElement]);
+
+    diffElement.loggedIn = false;
+    diffElement.path = 'some/path.ts';
+    const promise = mockPromise();
+    const setupDone = () => {
+      cursor._updateStops();
+      cursor.moveToFirstChunk();
+      diffElement.removeEventListener('render', setupDone);
+      promise.resolve();
+    };
+    diffElement.addEventListener('render', setupDone);
+
+    diff = createDiff();
+    diffElement.prefs = createDefaultDiffPrefs();
+    diffElement.diff = diff;
+    await promise;
+  });
+
+  test('diff cursor functionality (side-by-side)', () => {
+    // The cursor has been initialized to the first delta.
+    assert.isOk(cursor.diffRow);
+
+    const firstDeltaRow = queryAndAssert<HTMLElement>(
+      diffElement,
+      '.section.delta .diff-row'
+    );
+    assert.equal(cursor.diffRow, firstDeltaRow);
+
+    cursor.moveDown();
+
+    assert.isOk(firstDeltaRow.nextElementSibling);
+    assert.notEqual(cursor.diffRow, firstDeltaRow);
+    assert.equal(
+      cursor.diffRow,
+      firstDeltaRow.nextElementSibling as HTMLElement
+    );
+
+    cursor.moveUp();
+
+    assert.isOk(firstDeltaRow.nextElementSibling);
+    assert.notEqual(
+      cursor.diffRow,
+      firstDeltaRow.nextElementSibling as HTMLElement
+    );
+    assert.equal(cursor.diffRow, firstDeltaRow);
+  });
+
+  test('moveToFirstChunk', async () => {
+    const diff: DiffInfo = {
+      meta_a: {
+        name: 'lorem-ipsum.txt',
+        content_type: 'text/plain',
+        lines: 3,
+      },
+      meta_b: {
+        name: 'lorem-ipsum.txt',
+        content_type: 'text/plain',
+        lines: 3,
+      },
+      intraline_status: 'OK',
+      change_type: 'MODIFIED',
+      diff_header: [
+        'diff --git a/lorem-ipsum.txt b/lorem-ipsum.txt',
+        'index b2adcf4..554ae49 100644',
+        '--- a/lorem-ipsum.txt',
+        '+++ b/lorem-ipsum.txt',
+      ],
+      content: [
+        {b: ['new line 1']},
+        {ab: ['unchanged line']},
+        {a: ['old line 2']},
+        {ab: ['more unchanged lines']},
+      ],
+    };
+
+    diffElement.diff = diff;
+    // The file comment button, if present, is a cursor stop. Ensure
+    // moveToFirstChunk() works correctly even if the button is not shown.
+    diffElement.prefs!.show_file_comment_button = false;
+    await waitForEventOnce(diffElement, 'render');
+
+    cursor._updateStops();
+
+    const chunks = [
+      ...queryAll(diffElement, '.section.delta'),
+    ] as HTMLElement[];
+    assert.equal(chunks.length, 2);
+
+    // Verify it works on fresh diff.
+    cursor.moveToFirstChunk();
+    assert.ok(cursor.diffRow);
+    assert.equal(chunks.indexOf(cursor.diffRow!.parentElement!), 0);
+    assert.equal(cursor.side, Side.RIGHT);
+
+    // Verify it works from other cursor positions.
+    cursor.moveToNextChunk();
+    assert.ok(cursor.diffRow);
+    assert.equal(chunks.indexOf(cursor.diffRow!.parentElement!), 1);
+    assert.equal(cursor.side, Side.LEFT);
+    cursor.moveToFirstChunk();
+    assert.ok(cursor.diffRow);
+    assert.equal(chunks.indexOf(cursor.diffRow!.parentElement!), 0);
+    assert.equal(cursor.side, Side.RIGHT);
+  });
+
+  test('moveToLastChunk', async () => {
+    const diff: DiffInfo = {
+      meta_a: {
+        name: 'lorem-ipsum.txt',
+        content_type: 'text/plain',
+        lines: 3,
+      },
+      meta_b: {
+        name: 'lorem-ipsum.txt',
+        content_type: 'text/plain',
+        lines: 3,
+      },
+      intraline_status: 'OK',
+      change_type: 'MODIFIED',
+      diff_header: [
+        'diff --git a/lorem-ipsum.txt b/lorem-ipsum.txt',
+        'index b2adcf4..554ae49 100644',
+        '--- a/lorem-ipsum.txt',
+        '+++ b/lorem-ipsum.txt',
+      ],
+      content: [
+        {ab: ['unchanged line']},
+        {a: ['old line 2']},
+        {ab: ['more unchanged lines']},
+        {b: ['new line 3']},
+      ],
+    };
+
+    diffElement.diff = diff;
+    await waitForEventOnce(diffElement, 'render');
+    cursor._updateStops();
+
+    const chunks = [...queryAll(diffElement, '.section.delta')];
+    assert.equal(chunks.length, 2);
+
+    // Verify it works on fresh diff.
+    cursor.moveToLastChunk();
+    assert.equal(chunks.indexOf(cursor.diffRow!.parentElement!), 1);
+    assert.equal(cursor.side, Side.RIGHT);
+
+    // Verify it works from other cursor positions.
+    cursor.moveToPreviousChunk();
+    assert.equal(chunks.indexOf(cursor.diffRow!.parentElement!), 0);
+    assert.equal(cursor.side, Side.LEFT);
+    cursor.moveToLastChunk();
+    assert.equal(chunks.indexOf(cursor.diffRow!.parentElement!), 1);
+    assert.equal(cursor.side, Side.RIGHT);
+  });
+
+  test('cursor scroll behavior', () => {
+    assert.equal(cursor.cursorManager.scrollMode, 'keep-visible');
+
+    diffElement.dispatchEvent(new Event('render-start'));
+    assert.isTrue(cursor.cursorManager.focusOnMove);
+
+    window.dispatchEvent(new Event('scroll'));
+    assert.equal(cursor.cursorManager.scrollMode, 'never');
+    assert.isFalse(cursor.cursorManager.focusOnMove);
+
+    diffElement.dispatchEvent(new Event('render-content'));
+    assert.isTrue(cursor.cursorManager.focusOnMove);
+
+    cursor.reInitCursor();
+    assert.equal(cursor.cursorManager.scrollMode, 'keep-visible');
+  });
+
+  test('moves to selected line', () => {
+    const moveToNumStub = sinon.stub(cursor, 'moveToLineNumber');
+
+    diffElement.dispatchEvent(
+      new CustomEvent('line-selected', {
+        detail: {number: '123', side: Side.RIGHT, path: 'some/file'},
+      })
+    );
+
+    assert.isTrue(moveToNumStub.called);
+    assert.equal(moveToNumStub.lastCall.args[0], 123);
+    assert.equal(moveToNumStub.lastCall.args[1], Side.RIGHT);
+    assert.equal(moveToNumStub.lastCall.args[2], 'some/file');
+  });
+
+  suite('unified diff', () => {
+    setup(async () => {
+      diffElement.viewMode = DiffViewMode.UNIFIED;
+      await waitForEventOnce(diffElement, 'render');
+      cursor.reInitCursor();
+    });
+
+    test('diff cursor functionality (unified)', () => {
+      // The cursor has been initialized to the first delta.
+      assert.isOk(cursor.diffRow);
+
+      const firstDeltaRow = queryAndAssert<HTMLElement>(
+        diffElement,
+        '.section.delta .diff-row'
+      );
+      assert.equal(cursor.diffRow, firstDeltaRow);
+
+      cursor.moveDown();
+
+      assert.notEqual(cursor.diffRow, firstDeltaRow);
+      assert.equal(
+        cursor.diffRow,
+        firstDeltaRow.nextElementSibling as HTMLElement
+      );
+
+      cursor.moveUp();
+
+      assert.notEqual(
+        cursor.diffRow,
+        firstDeltaRow.nextElementSibling as HTMLElement
+      );
+      assert.equal(cursor.diffRow, firstDeltaRow);
+    });
+  });
+
+  test('cursor side functionality', () => {
+    // The side only applies to side-by-side mode, which should be the default
+    // mode.
+    assert.equal(diffElement.viewMode, 'SIDE_BY_SIDE');
+
+    const firstDeltaSection = queryAndAssert<HTMLElement>(
+      diffElement,
+      '.section.delta'
+    );
+    const firstDeltaRow = queryAndAssert<HTMLElement>(
+      firstDeltaSection,
+      '.diff-row'
+    );
+
+    // Because the first delta in this diff is on the right, it should be set
+    // to the right side.
+    assert.equal(cursor.side, Side.RIGHT);
+    assert.equal(cursor.diffRow, firstDeltaRow);
+    const firstIndex = cursor.cursorManager.index;
+
+    // Move the side to the left. Because this delta only has a right side, we
+    // should be moved up to the previous line where there is content on the
+    // right. The previous row is part of the previous section.
+    cursor.moveLeft();
+
+    assert.equal(cursor.side, Side.LEFT);
+    assert.notEqual(cursor.diffRow, firstDeltaRow);
+    assert.equal(cursor.cursorManager.index, firstIndex - 1);
+    assert.equal(
+      cursor.diffRow!.parentElement,
+      firstDeltaSection.previousSibling
+    );
+
+    // If we move down, we should skip everything in the first delta because
+    // we are on the left side and the first delta has no content on the left.
+    cursor.moveDown();
+
+    assert.equal(cursor.side, Side.LEFT);
+    assert.notEqual(cursor.diffRow, firstDeltaRow);
+    assert.isTrue(cursor.cursorManager.index > firstIndex);
+    assert.equal(cursor.diffRow!.parentElement, firstDeltaSection.nextSibling);
+  });
+
+  test('chunk skip functionality', () => {
+    const chunks = [...queryAll(diffElement, '.section.delta')];
+    const indexOfChunk = function (chunk: HTMLElement) {
+      return Array.prototype.indexOf.call(chunks, chunk);
+    };
+
+    // We should be initialized to the first chunk. Since this chunk only has
+    // content on the right side, our side should be right.
+    let currentIndex = indexOfChunk(cursor.diffRow!.parentElement!);
+    assert.equal(currentIndex, 0);
+    assert.equal(cursor.side, Side.RIGHT);
+
+    // Move to the next chunk.
+    cursor.moveToNextChunk();
+
+    // Since this chunk only has content on the left side. we should have been
+    // automatically moved over.
+    const previousIndex = currentIndex;
+    currentIndex = indexOfChunk(cursor.diffRow!.parentElement!);
+    assert.equal(currentIndex, previousIndex + 1);
+    assert.equal(cursor.side, Side.LEFT);
+  });
+
+  suite('moved chunks without line range)', () => {
+    setup(async () => {
+      const promise = mockPromise();
+      const renderHandler = function () {
+        diffElement.removeEventListener('render', renderHandler);
+        cursor.reInitCursor();
+        promise.resolve();
+      };
+      diffElement.addEventListener('render', renderHandler);
+      diffElement.diff = {
+        ...diff,
+        content: [
+          {
+            ab: ['Lorem ipsum dolor sit amet, suspendisse inceptos vehicula, '],
+          },
+          {
+            b: [
+              'Nullam neque, ligula ac, id blandit.',
+              'Sagittis tincidunt torquent, tempor nunc amet.',
+              'At rhoncus id.',
+            ],
+            move_details: {changed: false},
+          },
+          {
+            ab: ['Sem nascetur, erat ut, non in.'],
+          },
+          {
+            a: [
+              'Nullam neque, ligula ac, id blandit.',
+              'Sagittis tincidunt torquent, tempor nunc amet.',
+              'At rhoncus id.',
+            ],
+            move_details: {changed: false},
+          },
+          {
+            ab: ['Arcu eget, rhoncus amet cursus, ipsum elementum.'],
+          },
+        ],
+      };
+      await promise;
+    });
+
+    test('renders moveControls with simple descriptions', () => {
+      const [movedIn, movedOut] = [
+        ...queryAll(diffElement, '.dueToMove .moveControls'),
+      ];
+      assert.equal(movedIn.textContent, 'Moved in');
+      assert.equal(movedOut.textContent, 'Moved out');
+    });
+  });
+
+  suite('moved chunks (moveDetails)', () => {
+    setup(async () => {
+      const promise = mockPromise();
+      const renderHandler = function () {
+        diffElement.removeEventListener('render', renderHandler);
+        cursor.reInitCursor();
+        promise.resolve();
+      };
+      diffElement.addEventListener('render', renderHandler);
+      diffElement.diff = {
+        ...diff,
+        content: [
+          {
+            ab: ['Lorem ipsum dolor sit amet, suspendisse inceptos vehicula, '],
+          },
+          {
+            b: [
+              'Nullam neque, ligula ac, id blandit.',
+              'Sagittis tincidunt torquent, tempor nunc amet.',
+              'At rhoncus id.',
+            ],
+            move_details: {changed: false, range: {start: 4, end: 6}},
+          },
+          {
+            ab: ['Sem nascetur, erat ut, non in.'],
+          },
+          {
+            a: [
+              'Nullam neque, ligula ac, id blandit.',
+              'Sagittis tincidunt torquent, tempor nunc amet.',
+              'At rhoncus id.',
+            ],
+            move_details: {changed: false, range: {start: 2, end: 4}},
+          },
+          {
+            ab: ['Arcu eget, rhoncus amet cursus, ipsum elementum.'],
+          },
+        ],
+      };
+      await promise;
+    });
+
+    test('renders moveControls with simple descriptions', () => {
+      const [movedIn, movedOut] = [
+        ...queryAll(diffElement, '.dueToMove .moveControls'),
+      ];
+      assert.equal(movedIn.textContent, 'Moved from lines 4 - 6');
+      assert.equal(movedOut.textContent, 'Moved to lines 2 - 4');
+    });
+
+    test('startLineAnchor of movedIn chunk fires events', async () => {
+      const [movedIn] = [...queryAll(diffElement, '.dueToMove .moveControls')];
+      const [startLineAnchor] = movedIn.querySelectorAll('a');
+
+      const promise = mockPromise();
+      const onMovedLinkClicked = (e: CustomEvent) => {
+        assert.deepEqual(e.detail, {lineNum: 4, side: Side.LEFT});
+        promise.resolve();
+      };
+      assert.equal(startLineAnchor.textContent, '4');
+      startLineAnchor.addEventListener(
+        'moved-link-clicked',
+        onMovedLinkClicked
+      );
+      startLineAnchor.click();
+      await promise;
+    });
+
+    test('endLineAnchor of movedOut fires events', async () => {
+      const [, movedOut] = [
+        ...queryAll(diffElement, '.dueToMove .moveControls'),
+      ];
+      const [, endLineAnchor] = movedOut.querySelectorAll('a');
+
+      const promise = mockPromise();
+      const onMovedLinkClicked = (e: CustomEvent) => {
+        assert.deepEqual(e.detail, {lineNum: 4, side: Side.RIGHT});
+        promise.resolve();
+      };
+      assert.equal(endLineAnchor.textContent, '4');
+      endLineAnchor.addEventListener('moved-link-clicked', onMovedLinkClicked);
+      endLineAnchor.click();
+      await promise;
+    });
+  });
+
+  test('initialLineNumber not provided', async () => {
+    let scrollBehaviorDuringMove;
+    const moveToNumStub = sinon.stub(cursor, 'moveToLineNumber');
+    const moveToChunkStub = sinon
+      .stub(cursor, 'moveToFirstChunk')
+      .callsFake(() => {
+        scrollBehaviorDuringMove = cursor.cursorManager.scrollMode;
+      });
+
+    diffElement._diffChanged(createDiff());
+    await waitForEventOnce(diffElement, 'render');
+    cursor.reInitCursor();
+    assert.isFalse(moveToNumStub.called);
+    assert.isTrue(moveToChunkStub.called);
+    assert.equal(scrollBehaviorDuringMove, 'never');
+    assert.equal(cursor.cursorManager.scrollMode, 'keep-visible');
+  });
+
+  test('initialLineNumber provided', async () => {
+    let scrollBehaviorDuringMove;
+    const moveToNumStub = sinon
+      .stub(cursor, 'moveToLineNumber')
+      .callsFake(() => {
+        scrollBehaviorDuringMove = cursor.cursorManager.scrollMode;
+      });
+    const moveToChunkStub = sinon.stub(cursor, 'moveToFirstChunk');
+    cursor.initialLineNumber = 10;
+    cursor.side = Side.RIGHT;
+
+    diffElement._diffChanged(createDiff());
+    await waitForEventOnce(diffElement, 'render');
+    cursor.reInitCursor();
+    assert.isFalse(moveToChunkStub.called);
+    assert.isTrue(moveToNumStub.called);
+    assert.equal(moveToNumStub.lastCall.args[0], 10);
+    assert.equal(moveToNumStub.lastCall.args[1], Side.RIGHT);
+    assert.equal(scrollBehaviorDuringMove, 'keep-visible');
+    assert.equal(cursor.cursorManager.scrollMode, 'keep-visible');
+  });
+
+  test('getTargetDiffElement', () => {
+    cursor.initialLineNumber = 1;
+    assert.isTrue(!!cursor.diffRow);
+    assert.equal(cursor.getTargetDiffElement(), diffElement);
+  });
+
+  suite('createCommentInPlace', () => {
+    setup(() => {
+      diffElement.loggedIn = true;
+    });
+
+    test('adds new draft for selected line on the left', async () => {
+      cursor.moveToLineNumber(2, Side.LEFT);
+      const promise = mockPromise();
+      diffElement.addEventListener('create-comment', e => {
+        const {lineNum, range, side} = e.detail;
+        assert.equal(lineNum, 2);
+        assert.equal(range, undefined);
+        assert.equal(side, Side.LEFT);
+        promise.resolve();
+      });
+      cursor.createCommentInPlace();
+      await promise;
+    });
+
+    test('adds draft for selected line on the right', async () => {
+      cursor.moveToLineNumber(4, Side.RIGHT);
+      const promise = mockPromise();
+      diffElement.addEventListener('create-comment', e => {
+        const {lineNum, range, side} = e.detail;
+        assert.equal(lineNum, 4);
+        assert.equal(range, undefined);
+        assert.equal(side, Side.RIGHT);
+        promise.resolve();
+      });
+      cursor.createCommentInPlace();
+      await promise;
+    });
+
+    test('creates comment for range if selected', async () => {
+      const someRange = {
+        start_line: 2,
+        start_character: 3,
+        end_line: 6,
+        end_character: 1,
+      };
+      diffElement.highlights.selectedRange = {
+        side: Side.RIGHT,
+        range: someRange,
+      };
+      const promise = mockPromise();
+      diffElement.addEventListener('create-comment', e => {
+        const {lineNum, range, side} = e.detail;
+        assert.equal(lineNum, 6);
+        assert.equal(range, someRange);
+        assert.equal(side, Side.RIGHT);
+        promise.resolve();
+      });
+      cursor.createCommentInPlace();
+      await promise;
+    });
+
+    test('ignores call if nothing is selected', () => {
+      const createRangeCommentStub = sinon.stub(
+        diffElement,
+        'createRangeComment'
+      );
+      const addDraftAtLineStub = sinon.stub(diffElement, 'addDraftAtLine');
+      cursor.diffRow = undefined;
+      cursor.createCommentInPlace();
+      assert.isFalse(createRangeCommentStub.called);
+      assert.isFalse(addDraftAtLineStub.called);
+    });
+  });
+
+  test('getAddress', () => {
+    // It should initialize to the first chunk: line 5 of the revision.
+    assert.deepEqual(cursor.getAddress(), {leftSide: false, number: 5});
+
+    // Revision line 4 is up.
+    cursor.moveUp();
+    assert.deepEqual(cursor.getAddress(), {leftSide: false, number: 4});
+
+    // Base line 4 is left.
+    cursor.moveLeft();
+    assert.deepEqual(cursor.getAddress(), {leftSide: true, number: 4});
+
+    // Moving to the next chunk takes it back to the start.
+    cursor.moveToNextChunk();
+    assert.deepEqual(cursor.getAddress(), {leftSide: false, number: 5});
+
+    // The following chunk is a removal starting on line 10 of the base.
+    cursor.moveToNextChunk();
+    assert.deepEqual(cursor.getAddress(), {leftSide: true, number: 10});
+
+    // Should be null if there is no selection.
+    cursor.cursorManager.unsetCursor();
+    assert.isNotOk(cursor.getAddress());
+  });
+
+  test('_findRowByNumberAndFile', () => {
+    // Get the first ab row after the first chunk.
+    const rows = [...queryAll<HTMLTableRowElement>(diffElement, 'tr')];
+    const row = rows[9];
+    assert.ok(row);
+
+    // It should be line 8 on the right, but line 5 on the left.
+    assert.equal(cursor._findRowByNumberAndFile(8, Side.RIGHT), row);
+    assert.equal(cursor._findRowByNumberAndFile(5, Side.LEFT), row);
+  });
+
+  test('expand context updates stops', async () => {
+    const spy = sinon.spy(cursor, '_updateStops');
+    const controls = queryAndAssert(diffElement, 'gr-context-controls');
+    const showContext = queryAndAssert<HTMLElement>(controls, '.showContext');
+    showContext.click();
+    await waitForEventOnce(diffElement, 'render');
+    assert.isTrue(spy.called);
+  });
+
+  test('updates stops when loading changes', () => {
+    const spy = sinon.spy(cursor, '_updateStops');
+    diffElement.dispatchEvent(new Event('loading-changed'));
+    assert.isTrue(spy.called);
+  });
+
+  suite('multi diff', () => {
+    let diffElements: GrDiff[];
+
+    setup(async () => {
+      diffElements = [
+        await fixture(html`<gr-diff></gr-diff>`),
+        await fixture(html`<gr-diff></gr-diff>`),
+        await fixture(html`<gr-diff></gr-diff>`),
+      ];
+      cursor = new GrDiffCursor();
+
+      // Register the diff with the cursor.
+      cursor.replaceDiffs(diffElements);
+
+      for (const el of diffElements) {
+        el.prefs = createDefaultDiffPrefs();
+      }
+    });
+
+    function getTargetDiffIndex() {
+      // Mocha has a bug where when `assert.equals` fails, it will try to
+      // JSON.stringify the operands, which fails when they are cyclic structures
+      // like GrDiffElement. The failure is difficult to attribute to a specific
+      // assertion because of the async nature assertion errors are handled and
+      // can cause the test simply timing out, causing a lot of debugging headache.
+      // Working with indices circumvents the problem.
+      const target = cursor.getTargetDiffElement();
+      assertIsDefined(target);
+      return diffElements.indexOf(target);
+    }
+
+    test('do not skip loading diffs', async () => {
+      diffElements[0].diff = createDiff();
+      diffElements[2].diff = createDiff();
+      await waitForEventOnce(diffElements[0], 'render');
+      await waitForEventOnce(diffElements[2], 'render');
+
+      const lastLine = diffElements[0].diff.meta_b?.lines;
+      assertIsDefined(lastLine);
+
+      // Goto second last line of the first diff
+      cursor.moveToLineNumber(lastLine - 1, Side.RIGHT);
+      assert.equal(
+        cursor.getTargetLineElement()!.textContent,
+        `${lastLine - 1}`
+      );
+
+      // Can move down until we reach the loading file
+      cursor.moveDown();
+      assert.equal(getTargetDiffIndex(), 0);
+      assert.equal(
+        cursor.getTargetLineElement()!.textContent,
+        lastLine.toString()
+      );
+
+      // Cannot move down while still loading the diff we would switch to
+      cursor.moveDown();
+      assert.equal(getTargetDiffIndex(), 0);
+      assert.equal(
+        cursor.getTargetLineElement()!.textContent,
+        lastLine.toString()
+      );
+
+      // Diff 1 finishing to load
+      diffElements[1].diff = createDiff();
+      await waitForEventOnce(diffElements[1], 'render');
+
+      // Now we can go down
+      cursor.moveDown();
+      assert.equal(getTargetDiffIndex(), 1);
+      assert.equal(cursor.getTargetLineElement()!.textContent, 'File');
+    });
+  });
+});
diff --git a/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-group_test.js b/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-group_test.ts
similarity index 62%
rename from polygerrit-ui/app/embed/diff/gr-diff/gr-diff-group_test.js
rename to polygerrit-ui/app/embed/diff/gr-diff/gr-diff-group_test.ts
index 321086c..43a56d1 100644
--- a/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-group_test.js
+++ b/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-group_test.ts
@@ -1,32 +1,25 @@
 /**
  * @license
- * Copyright (C) 2016 The Android Open Source Project
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- * http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
+ * Copyright 2016 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
  */
-
-import '../../../test/common-test-setup-karma.js';
-import {GrDiffLine, GrDiffLineType, BLANK_LINE} from './gr-diff-line.js';
-import {GrDiffGroup, GrDiffGroupType, hideInContextControl} from './gr-diff-group.js';
+import '../../../test/common-test-setup-karma';
+import {GrDiffLine, GrDiffLineType, BLANK_LINE} from './gr-diff-line';
+import {
+  GrDiffGroup,
+  GrDiffGroupType,
+  hideInContextControl,
+} from './gr-diff-group';
 
 suite('gr-diff-group tests', () => {
   test('delta line pairs', () => {
     const l1 = new GrDiffLine(GrDiffLineType.ADD, 0, 128);
     const l2 = new GrDiffLine(GrDiffLineType.ADD, 0, 129);
     const l3 = new GrDiffLine(GrDiffLineType.REMOVE, 64, 0);
-    let group = new GrDiffGroup({type: GrDiffGroupType.DELTA, lines: [
-      l1, l2, l3,
-    ]});
+    let group = new GrDiffGroup({
+      type: GrDiffGroupType.DELTA,
+      lines: [l1, l2, l3],
+    });
     assert.deepEqual(group.lines, [l1, l2, l3]);
     assert.deepEqual(group.adds, [l1, l2]);
     assert.deepEqual(group.removes, [l3]);
@@ -59,7 +52,9 @@
     const l3 = new GrDiffLine(GrDiffLineType.BOTH, 66, 130);
 
     const group = new GrDiffGroup({
-      type: GrDiffGroupType.BOTH, lines: [l1, l2, l3]});
+      type: GrDiffGroupType.BOTH,
+      lines: [l1, l2, l3],
+    });
 
     assert.deepEqual(group.lines, [l1, l2, l3]);
     assert.deepEqual(group.adds, []);
@@ -83,34 +78,44 @@
     const l2 = new GrDiffLine(GrDiffLineType.REMOVE);
     const l3 = new GrDiffLine(GrDiffLineType.BOTH);
 
-    assert.throws(() =>
-      new GrDiffGroup({type: GrDiffGroupType.BOTH, lines: [l1, l2, l3]}));
+    assert.throws(
+      () => new GrDiffGroup({type: GrDiffGroupType.BOTH, lines: [l1, l2, l3]})
+    );
   });
 
   suite('hideInContextControl', () => {
-    let groups;
+    let groups: GrDiffGroup[];
     setup(() => {
       groups = [
-        new GrDiffGroup({type: GrDiffGroupType.BOTH, lines: [
-          new GrDiffLine(GrDiffLineType.BOTH, 5, 7),
-          new GrDiffLine(GrDiffLineType.BOTH, 6, 8),
-          new GrDiffLine(GrDiffLineType.BOTH, 7, 9),
-        ]}),
-        new GrDiffGroup({type: GrDiffGroupType.DELTA, lines: [
-          new GrDiffLine(GrDiffLineType.REMOVE, 8),
-          new GrDiffLine(GrDiffLineType.ADD, 0, 10),
-          new GrDiffLine(GrDiffLineType.REMOVE, 9),
-          new GrDiffLine(GrDiffLineType.ADD, 0, 11),
-          new GrDiffLine(GrDiffLineType.REMOVE, 10),
-          new GrDiffLine(GrDiffLineType.ADD, 0, 12),
-          new GrDiffLine(GrDiffLineType.REMOVE, 11),
-          new GrDiffLine(GrDiffLineType.ADD, 0, 13),
-        ]}),
-        new GrDiffGroup({type: GrDiffGroupType.BOTH, lines: [
-          new GrDiffLine(GrDiffLineType.BOTH, 12, 14),
-          new GrDiffLine(GrDiffLineType.BOTH, 13, 15),
-          new GrDiffLine(GrDiffLineType.BOTH, 14, 16),
-        ]}),
+        new GrDiffGroup({
+          type: GrDiffGroupType.BOTH,
+          lines: [
+            new GrDiffLine(GrDiffLineType.BOTH, 5, 7),
+            new GrDiffLine(GrDiffLineType.BOTH, 6, 8),
+            new GrDiffLine(GrDiffLineType.BOTH, 7, 9),
+          ],
+        }),
+        new GrDiffGroup({
+          type: GrDiffGroupType.DELTA,
+          lines: [
+            new GrDiffLine(GrDiffLineType.REMOVE, 8),
+            new GrDiffLine(GrDiffLineType.ADD, 0, 10),
+            new GrDiffLine(GrDiffLineType.REMOVE, 9),
+            new GrDiffLine(GrDiffLineType.ADD, 0, 11),
+            new GrDiffLine(GrDiffLineType.REMOVE, 10),
+            new GrDiffLine(GrDiffLineType.ADD, 0, 12),
+            new GrDiffLine(GrDiffLineType.REMOVE, 11),
+            new GrDiffLine(GrDiffLineType.ADD, 0, 13),
+          ],
+        }),
+        new GrDiffGroup({
+          type: GrDiffGroupType.BOTH,
+          lines: [
+            new GrDiffLine(GrDiffLineType.BOTH, 12, 14),
+            new GrDiffLine(GrDiffLineType.BOTH, 13, 15),
+            new GrDiffLine(GrDiffLineType.BOTH, 14, 16),
+          ],
+        }),
       ];
     });
 
@@ -140,21 +145,25 @@
       assert.equal(collapsedGroups[2].contextGroups.length, 2);
 
       assert.equal(
-          collapsedGroups[2].contextGroups[0].type,
-          GrDiffGroupType.DELTA);
+        collapsedGroups[2].contextGroups[0].type,
+        GrDiffGroupType.DELTA
+      );
       assert.deepEqual(
-          collapsedGroups[2].contextGroups[0].adds,
-          groups[1].adds.slice(1));
+        collapsedGroups[2].contextGroups[0].adds,
+        groups[1].adds.slice(1)
+      );
       assert.deepEqual(
-          collapsedGroups[2].contextGroups[0].removes,
-          groups[1].removes.slice(1));
+        collapsedGroups[2].contextGroups[0].removes,
+        groups[1].removes.slice(1)
+      );
 
       assert.equal(
-          collapsedGroups[2].contextGroups[1].type,
-          GrDiffGroupType.BOTH);
-      assert.deepEqual(
-          collapsedGroups[2].contextGroups[1].lines,
-          [groups[2].lines[0]]);
+        collapsedGroups[2].contextGroups[1].type,
+        GrDiffGroupType.BOTH
+      );
+      assert.deepEqual(collapsedGroups[2].contextGroups[1].lines, [
+        groups[2].lines[0],
+      ]);
 
       assert.equal(collapsedGroups[3].type, GrDiffGroupType.BOTH);
       assert.deepEqual(collapsedGroups[3].lines, groups[2].lines.slice(1));
@@ -166,19 +175,26 @@
           type: GrDiffGroupType.BOTH,
           skip: 60,
           offsetLeft: 8,
-          offsetRight: 10});
+          offsetRight: 10,
+        });
         groups = [
-          new GrDiffGroup({type: GrDiffGroupType.BOTH, lines: [
-            new GrDiffLine(GrDiffLineType.BOTH, 5, 7),
-            new GrDiffLine(GrDiffLineType.BOTH, 6, 8),
-            new GrDiffLine(GrDiffLineType.BOTH, 7, 9),
-          ]}),
+          new GrDiffGroup({
+            type: GrDiffGroupType.BOTH,
+            lines: [
+              new GrDiffLine(GrDiffLineType.BOTH, 5, 7),
+              new GrDiffLine(GrDiffLineType.BOTH, 6, 8),
+              new GrDiffLine(GrDiffLineType.BOTH, 7, 9),
+            ],
+          }),
           skipGroup,
-          new GrDiffGroup({type: GrDiffGroupType.BOTH, lines: [
-            new GrDiffLine(GrDiffLineType.BOTH, 68, 70),
-            new GrDiffLine(GrDiffLineType.BOTH, 69, 71),
-            new GrDiffLine(GrDiffLineType.BOTH, 70, 72),
-          ]}),
+          new GrDiffGroup({
+            type: GrDiffGroupType.BOTH,
+            lines: [
+              new GrDiffLine(GrDiffLineType.BOTH, 68, 70),
+              new GrDiffLine(GrDiffLineType.BOTH, 69, 71),
+              new GrDiffLine(GrDiffLineType.BOTH, 70, 72),
+            ],
+          }),
         ];
       });
 
@@ -189,13 +205,11 @@
     });
 
     test('groups unchanged if the hidden range is empty', () => {
-      assert.deepEqual(
-          hideInContextControl(groups, 0, 0), groups);
+      assert.deepEqual(hideInContextControl(groups, 0, 0), groups);
     });
 
     test('groups unchanged if there is only 1 line to hide', () => {
-      assert.deepEqual(
-          hideInContextControl(groups, 3, 4), groups);
+      assert.deepEqual(hideInContextControl(groups, 3, 4), groups);
     });
   });
 
@@ -206,7 +220,7 @@
         lines.push(new GrDiffLine(GrDiffLineType.ADD));
       }
       const group = new GrDiffGroup({type: GrDiffGroupType.DELTA, lines});
-      assert.isTrue(group.isTotal(group));
+      assert.isTrue(group.isTotal());
     });
 
     test('is total for remove', () => {
@@ -215,12 +229,12 @@
         lines.push(new GrDiffLine(GrDiffLineType.REMOVE));
       }
       const group = new GrDiffGroup({type: GrDiffGroupType.DELTA, lines});
-      assert.isTrue(group.isTotal(group));
+      assert.isTrue(group.isTotal());
     });
 
     test('not total for empty', () => {
       const group = new GrDiffGroup({type: GrDiffGroupType.BOTH});
-      assert.isFalse(group.isTotal(group));
+      assert.isFalse(group.isTotal());
     });
 
     test('not total for non-delta', () => {
@@ -229,8 +243,7 @@
         lines.push(new GrDiffLine(GrDiffLineType.BOTH));
       }
       const group = new GrDiffGroup({type: GrDiffGroupType.DELTA, lines});
-      assert.isFalse(group.isTotal(group));
+      assert.isFalse(group.isTotal());
     });
   });
 });
-
diff --git a/polygerrit-ui/app/embed/diff/gr-diff/gr-diff.ts b/polygerrit-ui/app/embed/diff/gr-diff/gr-diff.ts
index e5f9de0..27952d3 100644
--- a/polygerrit-ui/app/embed/diff/gr-diff/gr-diff.ts
+++ b/polygerrit-ui/app/embed/diff/gr-diff/gr-diff.ts
@@ -43,11 +43,7 @@
 import {getHiddenScroll} from '../../../scripts/hiddenscroll';
 import {customElement, observe, property} from '@polymer/decorators';
 import {BlameInfo, CommentRange, ImageInfo} from '../../../types/common';
-import {
-  DiffInfo,
-  DiffPreferencesInfo,
-  DiffPreferencesInfoKey,
-} from '../../../types/diff';
+import {DiffInfo, DiffPreferencesInfo} from '../../../types/diff';
 import {GrDiffHighlight} from '../gr-diff-highlight/gr-diff-highlight';
 import {
   GrDiffBuilderElement,
@@ -81,6 +77,7 @@
 import {assertIsDefined} from '../../../utils/common-util';
 import {debounce, DelayedTask} from '../../../utils/async-util';
 import {GrDiffSelection} from '../gr-diff-selection/gr-diff-selection';
+import {deepEqual} from '../../../utils/deep-util';
 
 const NO_NEWLINE_LEFT = 'No newline at end of left file.';
 const NO_NEWLINE_RIGHT = 'No newline at end of right file.';
@@ -100,7 +97,6 @@
 
 export interface GrDiff {
   $: {
-    diffBuilder: GrDiffBuilderElement;
     diffTable: HTMLTableElement;
   };
 }
@@ -179,7 +175,7 @@
   @property({type: Object})
   highlightRange?: CommentRange;
 
-  @property({type: Array})
+  @property({type: Array, observer: '_coverageRangesObserver'})
   coverageRanges: CoverageRange[] = [];
 
   @property({type: Boolean, observer: '_lineWrappingObserver'})
@@ -248,9 +244,6 @@
   @property({type: Object, observer: '_blameChanged'})
   blame: BlameInfo[] | null = null;
 
-  @property({type: Number})
-  parentIndex?: number;
-
   @property({type: Boolean})
   showNewlineWarningLeft = false;
 
@@ -292,11 +285,16 @@
   @property({type: Boolean})
   isAttached = false;
 
-  private renderDiffTableTask?: DelayedTask;
+  // visible for testing
+  renderDiffTableTask?: DelayedTask;
 
   private diffSelection = new GrDiffSelection();
 
-  private highlights = new GrDiffHighlight();
+  // visible for testing
+  highlights = new GrDiffHighlight();
+
+  // visible for testing
+  diffBuilder = new GrDiffBuilderElement();
 
   constructor() {
     super();
@@ -321,11 +319,12 @@
     this._unobserveNodes();
     this.diffSelection.cleanup();
     this.highlights.cleanup();
+    this.diffBuilder.cancel();
     super.disconnectedCallback();
   }
 
   getLineNumEls(side: Side): HTMLElement[] {
-    return this.$.diffBuilder.getLineNumEls(side);
+    return this.diffBuilder.getLineNumEls(side);
   }
 
   showNoChangeMessage(
@@ -426,19 +425,25 @@
           cr.side === removedCommentRange.side &&
           rangesEqual(cr.range, removedCommentRange.range)
       );
-      this.splice('_commentRanges', i, 1);
+      this._commentRanges.splice(i, 1);
     }
 
-    if (addedCommentRanges && addedCommentRanges.length) {
-      this.push('_commentRanges', ...addedCommentRanges);
+    if (addedCommentRanges?.length) {
+      this._commentRanges.push(...addedCommentRanges);
     }
     if (this.highlightRange) {
-      this.push('_commentRanges', {
+      this._commentRanges.push({
         side: Side.RIGHT,
         range: this.highlightRange,
         rootId: '',
       });
     }
+
+    this.diffBuilder.updateCommentRanges(this._commentRanges);
+  }
+
+  _coverageRangesObserver() {
+    this.diffBuilder.updateCoverageRanges(this.coverageRanges);
   }
 
   /**
@@ -483,7 +488,7 @@
 
   /** Cancel any remaining diff builder rendering work. */
   cancel() {
-    this.$.diffBuilder.cancel();
+    this.diffBuilder.cancel();
     this.renderDiffTableTask?.cancel();
   }
 
@@ -492,7 +497,7 @@
 
     // Get rendered stops.
     const stops: Array<HTMLElement | AbortStop> =
-      this.$.diffBuilder.getLineNumberRows();
+      this.diffBuilder.getLineNumberRows();
 
     // If we are still loading this diff, abort after the rendered stops to
     // avoid skipping over to e.g. the next file.
@@ -512,7 +517,7 @@
 
   _blameChanged(newValue?: BlameInfo[] | null) {
     if (newValue === undefined) return;
-    this.$.diffBuilder.setBlame(newValue);
+    this.diffBuilder.setBlame(newValue);
     if (newValue) {
       this.classList.add('showBlame');
     } else {
@@ -534,7 +539,7 @@
     return classes.join(' ');
   }
 
-  _handleTap(e: CustomEvent) {
+  _handleTap(e: Event) {
     const el = (dom(e) as EventApi).localTarget as Element;
 
     if (
@@ -603,7 +608,7 @@
 
   _createCommentForSelection(side: Side, range: CommentRange) {
     const lineNum = range.end_line;
-    const lineEl = this.$.diffBuilder.getLineElByNumber(lineNum, side);
+    const lineEl = this.diffBuilder.getLineElByNumber(lineNum, side);
     if (lineEl) {
       this._createComment(lineEl, lineNum, side, range);
     }
@@ -621,7 +626,7 @@
     side?: Side,
     range?: CommentRange
   ) {
-    const contentEl = this.$.diffBuilder.getContentTdByLineEl(lineEl);
+    const contentEl = this.diffBuilder.getContentTdByLineEl(lineEl);
     if (!contentEl) throw new Error('content el not found for line el');
     side = side ?? this._getCommentSideByLineAndContent(lineEl, contentEl);
     assertIsDefined(this.path, 'path');
@@ -663,28 +668,11 @@
   }
 
   _prefsObserver(newPrefs: DiffPreferencesInfo, oldPrefs: DiffPreferencesInfo) {
-    if (!this._prefsEqual(newPrefs, oldPrefs)) {
+    if (!deepEqual(newPrefs, oldPrefs)) {
       this._prefsChanged(newPrefs);
     }
   }
 
-  _prefsEqual(prefs1: DiffPreferencesInfo, prefs2: DiffPreferencesInfo) {
-    if (prefs1 === prefs2) {
-      return true;
-    }
-    if (!prefs1 || !prefs2) {
-      return false;
-    }
-    // Scan the preference objects one level deep to see if they differ.
-    const keys1 = Object.keys(prefs1) as DiffPreferencesInfoKey[];
-    const keys2 = Object.keys(prefs2) as DiffPreferencesInfoKey[];
-    return (
-      keys1.length === keys2.length &&
-      keys1.every(key => prefs1[key] === prefs2[key]) &&
-      keys2.every(key => prefs1[key] === prefs2[key])
-    );
-  }
-
   _pathObserver() {
     // Call _prefsChanged(), because line-limit style value depends on path.
     this._prefsChanged(this.prefs);
@@ -699,7 +687,7 @@
     if (!this.lineOfInterest) return;
     const lineNum = this.lineOfInterest.lineNum;
     if (typeof lineNum !== 'number') return;
-    this.$.diffBuilder.unhideLine(lineNum, this.lineOfInterest.side);
+    this.diffBuilder.unhideLine(lineNum, this.lineOfInterest.side);
   }
 
   _cleanup() {
@@ -808,7 +796,7 @@
     if (this.prefs) {
       this._updatePreferenceStyles(this.prefs, renderPrefs);
     }
-    this.$.diffBuilder.updateRenderPrefs(renderPrefs);
+    this.diffBuilder.updateRenderPrefs(renderPrefs);
   }
 
   _diffChanged(newValue?: DiffInfo) {
@@ -820,7 +808,7 @@
     }
     if (this.diff) {
       this.diffSelection.init(this.diff, this.$.diffTable);
-      this.highlights.init(this.$.diffTable, this.$.diffBuilder);
+      this.highlights.init(this.$.diffTable, this.diffBuilder);
     }
   }
 
@@ -866,9 +854,24 @@
     this._showWarning = false;
 
     const keyLocations = this._computeKeyLocations();
-    this.$.diffBuilder.prefs = this._getBypassPrefs(this.prefs);
-    this.$.diffBuilder.renderPrefs = this.renderPrefs;
-    this.$.diffBuilder.render(keyLocations);
+
+    // TODO: Setting tons of public properties like this is obviously a code
+    // smell. We are planning to introduce a diff model for managing all this
+    // data. Then diff builder will only need access to that model.
+    this.diffBuilder.prefs = this._getBypassPrefs(this.prefs);
+    this.diffBuilder.renderPrefs = this.renderPrefs;
+    this.diffBuilder.diff = this.diff;
+    this.diffBuilder.path = this.path;
+    this.diffBuilder.viewMode = this.viewMode;
+    this.diffBuilder.layers = this.layers ?? [];
+    this.diffBuilder.isImageDiff = this.isImageDiff;
+    this.diffBuilder.baseImage = this.baseImage ?? null;
+    this.diffBuilder.revisionImage = this.revisionImage ?? null;
+    this.diffBuilder.useNewImageDiffUi = this.useNewImageDiffUi;
+    this.diffBuilder.diffElement = this.$.diffTable;
+    this.diffBuilder.updateCommentRanges(this._commentRanges);
+    this.diffBuilder.updateCoverageRanges(this.coverageRanges);
+    this.diffBuilder.render(keyLocations);
   }
 
   _handleRenderContent() {
@@ -895,10 +898,7 @@
         const commentSide = getSide(threadEl);
         const range = getRange(threadEl);
         if (!commentSide) continue;
-        const lineEl = this.$.diffBuilder.getLineElByNumber(
-          lineNum,
-          commentSide
-        );
+        const lineEl = this.diffBuilder.getLineElByNumber(lineNum, commentSide);
         // When the line the comment refers to does not exist, log an error
         // but don't crash. This can happen e.g. if the API does not fully
         // validate e.g. (robot) comments
@@ -911,7 +911,7 @@
           );
           continue;
         }
-        const contentEl = this.$.diffBuilder.getContentTdByLineEl(lineEl);
+        const contentEl = this.diffBuilder.getContentTdByLineEl(lineEl);
         if (!contentEl) continue;
         if (lineNum === 'LOST' && !contentEl.hasChildNodes()) {
           contentEl.appendChild(this._portedCommentsWithoutRangeMessage());
diff --git a/polygerrit-ui/app/embed/diff/gr-diff/gr-diff_html.ts b/polygerrit-ui/app/embed/diff/gr-diff/gr-diff_html.ts
index 6d36b89..40d4e7f 100644
--- a/polygerrit-ui/app/embed/diff/gr-diff/gr-diff_html.ts
+++ b/polygerrit-ui/app/embed/diff/gr-diff/gr-diff_html.ts
@@ -698,36 +698,22 @@
     class$="[[_computeContainerClass(loggedIn, viewMode, displayLine)]]"
     on-click="_handleTap"
   >
-    <gr-diff-builder
-      id="diffBuilder"
-      comment-ranges="[[_commentRanges]]"
-      coverage-ranges="[[coverageRanges]]"
-      diff="[[diff]]"
-      path="[[path]]"
-      view-mode="[[viewMode]]"
-      is-image-diff="[[isImageDiff]]"
-      base-image="[[baseImage]]"
-      layers="[[layers]]"
-      revision-image="[[revisionImage]]"
-      use-new-image-diff-ui="[[useNewImageDiffUi]]"
-    >
-      <table
-        id="diffTable"
-        class$="[[_diffTableClass]]"
-        role="presentation"
-        contenteditable$="[[isContentEditable]]"
-      ></table>
+    <table
+      id="diffTable"
+      class$="[[_diffTableClass]]"
+      role="presentation"
+      contenteditable$="[[isContentEditable]]"
+    ></table>
 
-      <template
-        is="dom-if"
-        if="[[showNoChangeMessage(_loading, prefs, _diffLength, diff)]]"
-      >
-        <div class="whitespace-change-only-message">
-          This file only contains whitespace changes. Modify the whitespace
-          setting to see the changes.
-        </div>
-      </template>
-    </gr-diff-builder>
+    <template
+      is="dom-if"
+      if="[[showNoChangeMessage(_loading, prefs, _diffLength, diff)]]"
+    >
+      <div class="whitespace-change-only-message">
+        This file only contains whitespace changes. Modify the whitespace
+        setting to see the changes.
+      </div>
+    </template>
   </div>
   <div class$="[[_computeNewlineWarningClass(_newlineWarning, _loading)]]">
     [[_newlineWarning]]
diff --git a/polygerrit-ui/app/embed/diff/gr-diff/gr-diff_test.js b/polygerrit-ui/app/embed/diff/gr-diff/gr-diff_test.ts
similarity index 62%
rename from polygerrit-ui/app/embed/diff/gr-diff/gr-diff_test.js
rename to polygerrit-ui/app/embed/diff/gr-diff/gr-diff_test.ts
index c8b643d..183cdfb 100644
--- a/polygerrit-ui/app/embed/diff/gr-diff/gr-diff_test.js
+++ b/polygerrit-ui/app/embed/diff/gr-diff/gr-diff_test.ts
@@ -1,32 +1,37 @@
 /**
  * @license
- * Copyright (C) 2015 The Android Open Source Project
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- * http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
+ * Copyright 2015 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
  */
-import '../../../test/common-test-setup-karma.js';
-import {createDiff} from '../../../test/test-data-generators.js';
-import './gr-diff.js';
-import {GrDiffBuilderImage} from '../gr-diff-builder/gr-diff-builder-image.js';
-import {getComputedStyleValue} from '../../../utils/dom-util.js';
-import {_setHiddenScroll} from '../../../scripts/hiddenscroll.js';
-import {runA11yAudit} from '../../../test/a11y-test-utils.js';
-import '@polymer/paper-button/paper-button.js';
-import {Side} from '../../../api/diff.js';
-import {mockPromise, stubRestApi} from '../../../test/test-utils.js';
-import {AbortStop} from '../../../api/core.js';
-import {afterNextRender} from '@polymer/polymer/lib/utils/render-status';
-import {waitForEventOnce} from '../../../utils/event-util.js';
+import '../../../test/common-test-setup-karma';
+import {createDiff} from '../../../test/test-data-generators';
+import './gr-diff';
+import {GrDiffBuilderImage} from '../gr-diff-builder/gr-diff-builder-image';
+import {getComputedStyleValue} from '../../../utils/dom-util';
+import {_setHiddenScroll} from '../../../scripts/hiddenscroll';
+import {runA11yAudit} from '../../../test/a11y-test-utils';
+import '@polymer/paper-button/paper-button';
+import {
+  DiffContent,
+  DiffInfo,
+  DiffPreferencesInfo,
+  DiffViewMode,
+  IgnoreWhitespaceType,
+  Side,
+} from '../../../api/diff';
+import {
+  mockPromise,
+  mouseDown,
+  query,
+  queryAll,
+  queryAndAssert,
+  stubRestApi,
+} from '../../../test/test-utils';
+import {AbortStop} from '../../../api/core';
+import {waitForEventOnce} from '../../../utils/event-util';
+import {GrDiff} from './gr-diff';
+import {ImageInfo} from '../../../types/common';
+import {GrRangedCommentHint} from '../gr-ranged-comment-hint/gr-ranged-comment-hint';
 
 const basicFixture = fixtureFromElement('gr-diff');
 
@@ -37,42 +42,51 @@
 });
 
 suite('gr-diff tests', () => {
-  let element;
+  let element: GrDiff;
 
-  const MINIMAL_PREFS = {tab_size: 2, line_length: 80, font_size: 12};
+  const MINIMAL_PREFS: DiffPreferencesInfo = {
+    tab_size: 2,
+    line_length: 80,
+    font_size: 12,
+    context: 3,
+    ignore_whitespace: 'IGNORE_NONE',
+  };
 
-  setup(() => {
-
-  });
+  setup(() => {});
 
   suite('selectionchange event handling', () => {
-    const emulateSelection = function() {
+    let handleSelectionChangeStub: sinon.SinonSpy;
+
+    const emulateSelection = function () {
       document.dispatchEvent(new CustomEvent('selectionchange'));
     };
 
     setup(() => {
       element = basicFixture.instantiate();
-      sinon.stub(element.highlights, 'handleSelectionChange');
+      handleSelectionChangeStub = sinon.spy(
+        element.highlights,
+        'handleSelectionChange'
+      );
     });
 
     test('enabled if logged in', async () => {
       element.loggedIn = true;
       emulateSelection();
       await flush();
-      assert.isTrue(element.highlights.handleSelectionChange.called);
+      assert.isTrue(handleSelectionChangeStub.called);
     });
 
     test('ignored if logged out', async () => {
       element.loggedIn = false;
       emulateSelection();
       await flush();
-      assert.isFalse(element.highlights.handleSelectionChange.called);
+      assert.isFalse(handleSelectionChangeStub.called);
     });
   });
 
   test('cancel', () => {
     element = basicFixture.instantiate();
-    const cancelStub = sinon.stub(element.$.diffBuilder, 'cancel');
+    const cancelStub = sinon.stub(element.diffBuilder, 'cancel');
     element.cancel();
     assert.isTrue(cancelStub.calledOnce);
   });
@@ -98,10 +112,12 @@
     });
 
     test('line limit is based on line_length', () => {
-      element.prefs = {...element.prefs, line_length: 100};
+      element.prefs = {...element.prefs!, line_length: 100};
       flush();
-      assert.equal(getComputedStyleValue('--line-limit-marker', element),
-          '100ch');
+      assert.equal(
+        getComputedStyleValue('--line-limit-marker', element),
+        '100ch'
+      );
     });
 
     test('content-width should not be defined', () => {
@@ -123,32 +139,40 @@
     });
 
     test('max-width considers two content columns in side-by-side', () => {
-      element.viewMode = 'SIDE_BY_SIDE';
+      element.viewMode = DiffViewMode.SIDE_BY_SIDE;
       flush();
-      assert.equal(getComputedStyleValue('--diff-max-width', element),
-          'calc(2 * 80ch + 2 * 48px + 0ch + 1px + 2px)');
+      assert.equal(
+        getComputedStyleValue('--diff-max-width', element),
+        'calc(2 * 80ch + 2 * 48px + 0ch + 1px + 2px)'
+      );
     });
 
     test('max-width considers one content column in unified', () => {
-      element.viewMode = 'UNIFIED_DIFF';
+      element.viewMode = DiffViewMode.UNIFIED;
       flush();
-      assert.equal(getComputedStyleValue('--diff-max-width', element),
-          'calc(1 * 80ch + 2 * 48px + 0ch + 1px + 2px)');
+      assert.equal(
+        getComputedStyleValue('--diff-max-width', element),
+        'calc(1 * 80ch + 2 * 48px + 0ch + 1px + 2px)'
+      );
     });
 
     test('max-width considers font-size', () => {
-      element.prefs = {...element.prefs, font_size: 13};
+      element.prefs = {...element.prefs!, font_size: 13};
       flush();
       // Each line number column: 4 * 13 = 52px
-      assert.equal(getComputedStyleValue('--diff-max-width', element),
-          'calc(2 * 80ch + 2 * 52px + 0ch + 1px + 2px)');
+      assert.equal(
+        getComputedStyleValue('--diff-max-width', element),
+        'calc(2 * 80ch + 2 * 52px + 0ch + 1px + 2px)'
+      );
     });
 
     test('sign cols are considered if show_sign_col is true', () => {
       element.renderPrefs = {...element.renderPrefs, show_sign_col: true};
       flush();
-      assert.equal(getComputedStyleValue('--diff-max-width', element),
-          'calc(2 * 80ch + 2 * 48px + 2ch + 1px + 2px)');
+      assert.equal(
+        getComputedStyleValue('--diff-max-width', element),
+        'calc(2 * 80ch + 2 * 48px + 2ch + 1px + 2px)'
+      );
     });
   });
 
@@ -168,39 +192,31 @@
     });
 
     test('view does not start with displayLine classList', () => {
-      assert.isFalse(
-          element.shadowRoot
-              .querySelector('.diffContainer')
-              .classList
-              .contains('displayLine'));
+      const container = queryAndAssert(element, '.diffContainer');
+      assert.isFalse(container.classList.contains('displayLine'));
     });
 
     test('displayLine class added called when displayLine is true', () => {
       const spy = sinon.spy(element, '_computeContainerClass');
       element.displayLine = true;
+      const container = queryAndAssert(element, '.diffContainer');
       assert.isTrue(spy.called);
-      assert.isTrue(
-          element.shadowRoot
-              .querySelector('.diffContainer')
-              .classList
-              .contains('displayLine'));
+      assert.isTrue(container.classList.contains('displayLine'));
     });
 
     test('thread groups', () => {
       const contentEl = document.createElement('div');
 
-      element.changeNum = 123;
-      element.patchRange = {basePatchNum: 1, patchNum: 2};
       element.path = 'file.txt';
-      element.$.diffBuilder.diff = createDiff();
-      element.$.diffBuilder.prefs = {...MINIMAL_PREFS};
-      element.$.diffBuilder._builder = element.$.diffBuilder._getDiffBuilder();
 
       // No thread groups.
       assert.equal(contentEl.querySelectorAll('.thread-group').length, 0);
 
       // A thread group gets created.
-      const threadGroupEl = element._getOrCreateThreadGroup(contentEl);
+      const threadGroupEl = element._getOrCreateThreadGroup(
+        contentEl,
+        Side.LEFT
+      );
       assert.isOk(threadGroupEl);
 
       // The new thread group can be fetched.
@@ -208,17 +224,19 @@
     });
 
     suite('image diffs', () => {
-      let mockFile1;
-      let mockFile2;
+      let mockFile1: ImageInfo;
+      let mockFile2: ImageInfo;
       setup(() => {
         mockFile1 = {
-          body: 'Qk06AAAAAAAAADYAAAAoAAAAAQAAAP////8BACAAAAAAAAAAAAATCwAAE' +
-          'wsAAAAAAAAAAAAAAAAA/w==',
+          body:
+            'Qk06AAAAAAAAADYAAAAoAAAAAQAAAP////8BACAAAAAAAAAAAAATCwAAE' +
+            'wsAAAAAAAAAAAAAAAAA/w==',
           type: 'image/bmp',
         };
         mockFile2 = {
-          body: 'Qk06AAAAAAAAADYAAAAoAAAAAQAAAP////8BACAAAAAAAAAAAAATCwAAE' +
-          'wsAAAAAAAAAAAAA/////w==',
+          body:
+            'Qk06AAAAAAAAADYAAAAoAAAAAQAAAP////8BACAAAAAAAAAAAAATCwAAE' +
+            'wsAAAAAAAAAAAAA/////w==',
           type: 'image/bmp',
         };
 
@@ -235,7 +253,6 @@
           show_whitespace_errors: true,
           syntax_highlighting: true,
           tab_size: 8,
-          theme: 'DEFAULT',
         };
       });
 
@@ -244,8 +261,7 @@
         element.revisionImage = mockFile2;
         element.diff = {
           meta_a: {name: 'carrot.jpg', content_type: 'image/jpeg', lines: 66},
-          meta_b: {name: 'carrot.jpg', content_type: 'image/jpeg',
-            lines: 560},
+          meta_b: {name: 'carrot.jpg', content_type: 'image/jpeg', lines: 560},
           intraline_status: 'OK',
           change_type: 'MODIFIED',
           diff_header: [
@@ -262,42 +278,40 @@
 
         // Recognizes that it should be an image diff.
         assert.isTrue(element.isImageDiff);
-        assert.instanceOf(
-            element.$.diffBuilder._builder, GrDiffBuilderImage);
+        assert.instanceOf(element.diffBuilder.builder, GrDiffBuilderImage);
 
         // Left image rendered with the parent commit's version of the file.
-        const leftImage = element.$.diffTable.querySelector('td.left img');
-        const leftLabel =
-              element.$.diffTable.querySelector('td.left label');
-        const leftLabelContent = leftLabel.querySelector('.label');
-        const leftLabelName = leftLabel.querySelector('.name');
+        const diffTable = element.$.diffTable;
+        const leftImage = queryAndAssert(diffTable, 'td.left img');
+        const leftLabel = queryAndAssert(diffTable, 'td.left label');
+        const leftLabelContent = queryAndAssert(leftLabel, '.label');
+        const leftLabelName = query(leftLabel, '.name');
 
-        const rightImage =
-              element.$.diffTable.querySelector('td.right img');
-        const rightLabel = element.$.diffTable.querySelector(
-            'td.right label');
-        const rightLabelContent = rightLabel.querySelector('.label');
-        const rightLabelName = rightLabel.querySelector('.name');
+        const rightImage = queryAndAssert(diffTable, 'td.right img');
+        const rightLabel = queryAndAssert(diffTable, 'td.right label');
+        const rightLabelContent = queryAndAssert(rightLabel, '.label');
+        const rightLabelName = query(rightLabel, '.name');
 
         assert.isNotOk(rightLabelName);
         assert.isNotOk(leftLabelName);
 
-        assert.isOk(leftImage);
-        assert.equal(leftImage.getAttribute('src'),
-            'data:image/bmp;base64,' + mockFile1.body);
-        assert.equal(leftLabelContent.textContent, '1\u00d71 image/bmp');// \u00d7 - '×'
+        assert.equal(
+          leftImage.getAttribute('src'),
+          'data:image/bmp;base64,' + mockFile1.body
+        );
+        assert.equal(leftLabelContent.textContent, '1\u00d71 image/bmp'); // \u00d7 - '×'
 
-        assert.isOk(rightImage);
-        assert.equal(rightImage.getAttribute('src'),
-            'data:image/bmp;base64,' + mockFile2.body);
-        assert.equal(rightLabelContent.textContent, '1\u00d71 image/bmp');// \u00d7 - '×'
+        assert.equal(
+          rightImage.getAttribute('src'),
+          'data:image/bmp;base64,' + mockFile2.body
+        );
+        assert.equal(rightLabelContent.textContent, '1\u00d71 image/bmp'); // \u00d7 - '×'
       });
 
       test('renders image diffs with a different file name', async () => {
-        const mockDiff = {
+        const mockDiff: DiffInfo = {
           meta_a: {name: 'carrot.jpg', content_type: 'image/jpeg', lines: 66},
-          meta_b: {name: 'carrot2.jpg', content_type: 'image/jpeg',
-            lines: 560},
+          meta_b: {name: 'carrot2.jpg', content_type: 'image/jpeg', lines: 560},
           intraline_status: 'OK',
           change_type: 'MODIFIED',
           diff_header: [
@@ -312,51 +326,51 @@
         };
 
         element.baseImage = mockFile1;
-        element.baseImage._name = mockDiff.meta_a.name;
+        element.baseImage._name = mockDiff.meta_a!.name;
         element.revisionImage = mockFile2;
-        element.revisionImage._name = mockDiff.meta_b.name;
+        element.revisionImage._name = mockDiff.meta_b!.name;
         element.diff = mockDiff;
         await waitForEventOnce(element, 'render');
 
         // Recognizes that it should be an image diff.
         assert.isTrue(element.isImageDiff);
-        assert.instanceOf(
-            element.$.diffBuilder._builder, GrDiffBuilderImage);
+        assert.instanceOf(element.diffBuilder.builder, GrDiffBuilderImage);
 
         // Left image rendered with the parent commit's version of the file.
-        const leftImage = element.$.diffTable.querySelector('td.left img');
-        const leftLabel =
-              element.$.diffTable.querySelector('td.left label');
-        const leftLabelContent = leftLabel.querySelector('.label');
-        const leftLabelName = leftLabel.querySelector('.name');
+        const diffTable = element.$.diffTable;
+        const leftImage = queryAndAssert(diffTable, 'td.left img');
+        const leftLabel = queryAndAssert(diffTable, 'td.left label');
+        const leftLabelContent = queryAndAssert(leftLabel, '.label');
+        const leftLabelName = queryAndAssert(leftLabel, '.name');
 
-        const rightImage =
-              element.$.diffTable.querySelector('td.right img');
-        const rightLabel = element.$.diffTable.querySelector(
-            'td.right label');
-        const rightLabelContent = rightLabel.querySelector('.label');
-        const rightLabelName = rightLabel.querySelector('.name');
+        const rightImage = queryAndAssert(diffTable, 'td.right img');
+        const rightLabel = queryAndAssert(diffTable, 'td.right label');
+        const rightLabelContent = queryAndAssert(rightLabel, '.label');
+        const rightLabelName = queryAndAssert(rightLabel, '.name');
 
         assert.isOk(rightLabelName);
         assert.isOk(leftLabelName);
-        assert.equal(leftLabelName.textContent, mockDiff.meta_a.name);
-        assert.equal(rightLabelName.textContent, mockDiff.meta_b.name);
+        assert.equal(leftLabelName.textContent, mockDiff.meta_a?.name);
+        assert.equal(rightLabelName.textContent, mockDiff.meta_b?.name);
 
         assert.isOk(leftImage);
-        assert.equal(leftImage.getAttribute('src'),
-            'data:image/bmp;base64,' + mockFile1.body);
-        assert.equal(leftLabelContent.textContent, '1\u00d71 image/bmp');// \u00d7 - '×'
+        assert.equal(
+          leftImage.getAttribute('src'),
+          'data:image/bmp;base64,' + mockFile1.body
+        );
+        assert.equal(leftLabelContent.textContent, '1\u00d71 image/bmp'); // \u00d7 - '×'
 
         assert.isOk(rightImage);
-        assert.equal(rightImage.getAttribute('src'),
-            'data:image/bmp;base64,' + mockFile2.body);
-        assert.equal(rightLabelContent.textContent, '1\u00d71 image/bmp');// \u00d7 - '×'
+        assert.equal(
+          rightImage.getAttribute('src'),
+          'data:image/bmp;base64,' + mockFile2.body
+        );
+        assert.equal(rightLabelContent.textContent, '1\u00d71 image/bmp'); // \u00d7 - '×'
       });
 
       test('renders added image', async () => {
-        const mockDiff = {
-          meta_b: {name: 'carrot.jpg', content_type: 'image/jpeg',
-            lines: 560},
+        const mockDiff: DiffInfo = {
+          meta_b: {name: 'carrot.jpg', content_type: 'image/jpeg', lines: 560},
           intraline_status: 'OK',
           change_type: 'ADDED',
           diff_header: [
@@ -371,7 +385,9 @@
         };
 
         const promise = mockPromise();
-        function rendered() { promise.resolve(); }
+        function rendered() {
+          promise.resolve();
+        }
         element.addEventListener('render', rendered);
 
         element.revisionImage = mockFile2;
@@ -380,20 +396,17 @@
         element.removeEventListener('render', rendered);
         // Recognizes that it should be an image diff.
         assert.isTrue(element.isImageDiff);
-        assert.instanceOf(
-            element.$.diffBuilder._builder, GrDiffBuilderImage);
+        assert.instanceOf(element.diffBuilder.builder, GrDiffBuilderImage);
 
-        const leftImage = element.$.diffTable.querySelector('td.left img');
-        const rightImage = element.$.diffTable.querySelector('td.right img');
-
+        const diffTable = element.$.diffTable;
+        const leftImage = query(diffTable, 'td.left img');
         assert.isNotOk(leftImage);
-        assert.isOk(rightImage);
+        queryAndAssert(diffTable, 'td.right img');
       });
 
       test('renders removed image', async () => {
-        const mockDiff = {
-          meta_a: {name: 'carrot.jpg', content_type: 'image/jpeg',
-            lines: 560},
+        const mockDiff: DiffInfo = {
+          meta_a: {name: 'carrot.jpg', content_type: 'image/jpeg', lines: 560},
           intraline_status: 'OK',
           change_type: 'DELETED',
           diff_header: [
@@ -407,7 +420,9 @@
           binary: true,
         };
         const promise = mockPromise();
-        function rendered() { promise.resolve(); }
+        function rendered() {
+          promise.resolve();
+        }
         element.addEventListener('render', rendered);
 
         element.baseImage = mockFile1;
@@ -416,20 +431,21 @@
         element.removeEventListener('render', rendered);
         // Recognizes that it should be an image diff.
         assert.isTrue(element.isImageDiff);
-        assert.instanceOf(
-            element.$.diffBuilder._builder, GrDiffBuilderImage);
+        assert.instanceOf(element.diffBuilder.builder, GrDiffBuilderImage);
 
-        const leftImage = element.$.diffTable.querySelector('td.left img');
-        const rightImage = element.$.diffTable.querySelector('td.right img');
-
-        assert.isOk(leftImage);
+        const diffTable = element.$.diffTable;
+        queryAndAssert(diffTable, 'td.left img');
+        const rightImage = query(diffTable, 'td.right img');
         assert.isNotOk(rightImage);
       });
 
       test('does not render disallowed image type', async () => {
-        const mockDiff = {
-          meta_a: {name: 'carrot.jpg', content_type: 'image/jpeg-evil',
-            lines: 560},
+        const mockDiff: DiffInfo = {
+          meta_a: {
+            name: 'carrot.jpg',
+            content_type: 'image/jpeg-evil',
+            lines: 560,
+          },
           intraline_status: 'OK',
           change_type: 'DELETED',
           diff_header: [
@@ -445,7 +461,9 @@
         mockFile1.type = 'image/jpeg-evil';
 
         const promise = mockPromise();
-        function rendered() { promise.resolve(); }
+        function rendered() {
+          promise.resolve();
+        }
         element.addEventListener('render', rendered);
 
         element.baseImage = mockFile1;
@@ -454,9 +472,9 @@
         element.removeEventListener('render', rendered);
         // Recognizes that it should be an image diff.
         assert.isTrue(element.isImageDiff);
-        assert.instanceOf(
-            element.$.diffBuilder._builder, GrDiffBuilderImage);
-        const leftImage = element.$.diffTable.querySelector('td.left img');
+        assert.instanceOf(element.diffBuilder.builder, GrDiffBuilderImage);
+        const diffTable = element.$.diffTable;
+        const leftImage = query(diffTable, 'td.left img');
         assert.isNotOk(leftImage);
       });
     });
@@ -513,7 +531,6 @@
           show_tabs: true,
           show_whitespace_errors: true,
           syntax_highlighting: true,
-          theme: 'DEFAULT',
           ignore_whitespace: 'IGNORE_NONE',
         };
 
@@ -548,20 +565,20 @@
         const FILE_ROW = 1;
         const actual = element.getCursorStops();
         assert.equal(actual.length, ROWS + FILE_ROW + 1);
-        assert.isTrue(actual[actual.length -1] instanceof AbortStop);
+        assert.isTrue(actual[actual.length - 1] instanceof AbortStop);
       });
     });
 
     test('adds .hiddenscroll', () => {
       _setHiddenScroll(true);
       element.displayLine = true;
-      assert.include(element.shadowRoot
-          .querySelector('.diffContainer').className, 'hiddenscroll');
+      const container = queryAndAssert(element, '.diffContainer');
+      assert.include(container.className, 'hiddenscroll');
     });
   });
 
   suite('logged in', () => {
-    let fakeLineEl;
+    let fakeLineEl: HTMLElement;
     setup(() => {
       element = basicFixture.instantiate();
       element.loggedIn = true;
@@ -571,15 +588,14 @@
         classList: {
           contains: sinon.stub().returns(true),
         },
-      };
+      } as unknown as HTMLElement;
     });
 
     test('addDraftAtLine', () => {
       sinon.stub(element, '_selectLine');
-      sinon.stub(element, '_createComment');
+      const createCommentStub = sinon.stub(element, '_createComment');
       element.addDraftAtLine(fakeLineEl);
-      assert.isTrue(element._createComment
-          .calledWithExactly(fakeLineEl, 42));
+      assert.isTrue(createCommentStub.calledWithExactly(fakeLineEl, 42));
     });
 
     test('adds long range comment hint', async () => {
@@ -592,23 +608,29 @@
       const threadEl = document.createElement('div');
       threadEl.className = 'comment-thread';
       threadEl.setAttribute('diff-side', 'right');
-      threadEl.setAttribute('line-num', 1);
+      threadEl.setAttribute('line-num', '1');
       threadEl.setAttribute('range', JSON.stringify(range));
       threadEl.setAttribute('slot', 'right-1');
-      const content = [{
-        a: [],
-        b: [],
-      }, {
-        ab: Array(13).fill('text'),
-      }];
+      const content = [
+        {
+          a: [],
+          b: [],
+        },
+        {
+          ab: Array(13).fill('text'),
+        },
+      ];
       setupSampleDiff({content});
-      await new Promise(resolve => afterNextRender(element, resolve));
+      await waitForEventOnce(element, 'render');
 
       element.appendChild(threadEl);
       await flush();
 
-      assert.deepEqual(
-          element.querySelector('gr-ranged-comment-hint').range, range);
+      const hint = queryAndAssert<GrRangedCommentHint>(
+        element,
+        'gr-ranged-comment-hint'
+      );
+      assert.deepEqual(hint.range, range);
     });
 
     test('no duplicate range hint for same thread', async () => {
@@ -621,19 +643,21 @@
       const threadEl = document.createElement('div');
       threadEl.className = 'comment-thread';
       threadEl.setAttribute('diff-side', 'right');
-      threadEl.setAttribute('line-num', 1);
+      threadEl.setAttribute('line-num', '1');
       threadEl.setAttribute('range', JSON.stringify(range));
       threadEl.setAttribute('slot', 'right-1');
       const firstHint = document.createElement('gr-ranged-comment-hint');
       firstHint.range = range;
-      firstHint.setAttribute('threadElRootId', threadEl.rootId);
       firstHint.setAttribute('slot', 'right-1');
-      const content = [{
-        a: [],
-        b: [],
-      }, {
-        ab: Array(13).fill('text'),
-      }];
+      const content = [
+        {
+          a: [],
+          b: [],
+        },
+        {
+          ab: Array(13).fill('text'),
+        },
+      ];
       setupSampleDiff({content});
 
       element.appendChild(firstHint);
@@ -644,86 +668,97 @@
       await flush();
 
       assert.equal(
-          element.querySelectorAll('gr-ranged-comment-hint').length, 1);
+        element.querySelectorAll('gr-ranged-comment-hint').length,
+        1
+      );
     });
 
-    test('removes long range comment hint when comment is discarded',
-        async () => {
-          const range = {
-            start_line: 1,
-            end_line: 7,
-            start_character: 0,
-            end_character: 0,
-          };
-          const threadEl = document.createElement('div');
-          threadEl.className = 'comment-thread';
-          threadEl.setAttribute('diff-side', 'right');
-          threadEl.setAttribute('line-num', 1);
-          threadEl.setAttribute('range', JSON.stringify(range));
-          threadEl.setAttribute('slot', 'right-1');
-          const content = [{
-            a: [],
-            b: [],
-          }, {
-            ab: Array(8).fill('text'),
-          }];
-          setupSampleDiff({content});
-          element.appendChild(threadEl);
-          await flush();
+    test('removes long range comment hint when comment is discarded', async () => {
+      const range = {
+        start_line: 1,
+        end_line: 7,
+        start_character: 0,
+        end_character: 0,
+      };
+      const threadEl = document.createElement('div');
+      threadEl.className = 'comment-thread';
+      threadEl.setAttribute('diff-side', 'right');
+      threadEl.setAttribute('line-num', '1');
+      threadEl.setAttribute('range', JSON.stringify(range));
+      threadEl.setAttribute('slot', 'right-1');
+      const content = [
+        {
+          a: [],
+          b: [],
+        },
+        {
+          ab: Array(8).fill('text'),
+        },
+      ];
+      setupSampleDiff({content});
+      element.appendChild(threadEl);
+      await flush();
 
-          threadEl.remove();
-          await flush();
+      threadEl.remove();
+      await flush();
 
-          assert.isEmpty(element.querySelectorAll('gr-ranged-comment-hint'));
-        });
+      assert.isEmpty(element.querySelectorAll('gr-ranged-comment-hint'));
+    });
 
     suite('change in preferences', () => {
       setup(() => {
         element.diff = {
           meta_a: {name: 'carrot.jpg', content_type: 'image/jpeg', lines: 66},
-          meta_b: {name: 'carrot.jpg', content_type: 'image/jpeg',
-            lines: 560},
+          meta_b: {name: 'carrot.jpg', content_type: 'image/jpeg', lines: 560},
           diff_header: [],
           intraline_status: 'OK',
           change_type: 'MODIFIED',
           content: [{skip: 66}],
         };
-        element.renderDiffTableTask.flush();
+        element.renderDiffTableTask?.flush();
       });
 
       test('change in preferences re-renders diff', () => {
-        sinon.stub(element, '_renderDiffTable');
+        const stub = sinon.stub(element, '_renderDiffTable');
         element.prefs = {
-          ...MINIMAL_PREFS, time_format: 'HHMM_12'};
-        element.renderDiffTableTask.flush();
-        assert.isTrue(element._renderDiffTable.called);
+          ...MINIMAL_PREFS,
+        };
+        element.renderDiffTableTask?.flush();
+        assert.isTrue(stub.called);
       });
 
       test('adding/removing property in preferences re-renders diff', () => {
         const stub = sinon.stub(element, '_renderDiffTable');
-        const newPrefs1 = {...MINIMAL_PREFS,
-          line_wrapping: true};
+        const newPrefs1: DiffPreferencesInfo = {
+          ...MINIMAL_PREFS,
+          line_wrapping: true,
+        };
         element.prefs = newPrefs1;
-        element.renderDiffTableTask.flush();
-        assert.isTrue(element._renderDiffTable.called);
+        element.renderDiffTableTask?.flush();
+        assert.isTrue(stub.called);
         stub.reset();
 
         const newPrefs2 = {...newPrefs1};
         delete newPrefs2.line_wrapping;
         element.prefs = newPrefs2;
-        element.renderDiffTableTask.flush();
-        assert.isTrue(element._renderDiffTable.called);
+        element.renderDiffTableTask?.flush();
+        assert.isTrue(stub.called);
       });
 
-      test('change in preferences does not re-renders diff with ' +
-          'noRenderOnPrefsChange', () => {
-        sinon.stub(element, '_renderDiffTable');
-        element.noRenderOnPrefsChange = true;
-        element.prefs = {
-          ...MINIMAL_PREFS, time_format: 'HHMM_12'};
-        element.renderDiffTableTask.flush();
-        assert.isFalse(element._renderDiffTable.called);
-      });
+      test(
+        'change in preferences does not re-renders diff with ' +
+          'noRenderOnPrefsChange',
+        () => {
+          const stub = sinon.stub(element, '_renderDiffTable');
+          element.noRenderOnPrefsChange = true;
+          element.prefs = {
+            ...MINIMAL_PREFS,
+            context: 12,
+          };
+          element.renderDiffTableTask?.flush();
+          assert.isFalse(stub.called);
+        }
+      );
     });
   });
 
@@ -732,8 +767,7 @@
       element = basicFixture.instantiate();
       element.diff = {
         meta_a: {name: 'carrot.jpg', content_type: 'image/jpeg', lines: 66},
-        meta_b: {name: 'carrot.jpg', content_type: 'image/jpeg',
-          lines: 560},
+        meta_b: {name: 'carrot.jpg', content_type: 'image/jpeg', lines: 560},
         diff_header: [],
         intraline_status: 'OK',
         change_type: 'MODIFIED',
@@ -755,11 +789,12 @@
       assert.equal(element._diffHeaderItems.length, 1);
       flush();
 
-      assert.equal(element.$.diffHeader.textContent.trim(), 'test');
+      const header = queryAndAssert(element, '#diffHeader');
+      assert.equal(header.textContent?.trim(), 'test');
     });
 
     test('binary files', () => {
-      element.diff.binary = true;
+      element.diff!.binary = true;
       assert.equal(element._diffHeaderItems.length, 0);
       element.push('diff.diff_header', 'diff --git a/test.jpg b/test.jpg');
       assert.equal(element._diffHeaderItems.length, 0);
@@ -771,16 +806,17 @@
   });
 
   suite('safety and bypass', () => {
-    let renderStub;
+    let renderStub: sinon.SinonStub;
 
     setup(() => {
       element = basicFixture.instantiate();
-      renderStub = sinon.stub(element.$.diffBuilder, 'render').callsFake(
-          () => {
-            element.$.diffBuilder.dispatchEvent(
-                new CustomEvent('render', {bubbles: true, composed: true}));
-            return Promise.resolve({});
-          });
+      renderStub = sinon.stub(element.diffBuilder, 'render').callsFake(() => {
+        const diffTable = element.$.diffTable;
+        diffTable.dispatchEvent(
+          new CustomEvent('render', {bubbles: true, composed: true})
+        );
+        return Promise.resolve({});
+      });
       sinon.stub(element, 'getDiffLength').returns(10000);
       element.diff = createDiff();
       element.noRenderOnPrefsChange = true;
@@ -838,7 +874,7 @@
 
       assert.equal(element.prefs.context, 3);
       assert.equal(element._safetyBypass, -1);
-      assert.equal(element.$.diffBuilder.prefs.context, -1);
+      assert.equal(element.diffBuilder.prefs.context, -1);
     });
 
     test('toggles collapse context from bypass', async () => {
@@ -851,7 +887,7 @@
 
       assert.equal(element.prefs.context, 3);
       assert.isNull(element._safetyBypass);
-      assert.equal(element.$.diffBuilder.prefs.context, 3);
+      assert.equal(element.diffBuilder.prefs.context, 3);
     });
 
     test('toggles collapse context from pref using default', async () => {
@@ -863,7 +899,7 @@
 
       assert.equal(element.prefs.context, -1);
       assert.equal(element._safetyBypass, 10);
-      assert.equal(element.$.diffBuilder.prefs.context, 10);
+      assert.equal(element.diffBuilder.prefs.context, 10);
     });
   });
 
@@ -874,7 +910,7 @@
 
     test('unsetting', () => {
       element.blame = [];
-      const setBlameSpy = sinon.spy(element.$.diffBuilder, 'setBlame');
+      const setBlameSpy = sinon.spy(element.diffBuilder, 'setBlame');
       element.classList.add('showBlame');
       element.blame = null;
       assert.isTrue(setBlameSpy.calledWithExactly(null));
@@ -882,7 +918,15 @@
     });
 
     test('setting', () => {
-      element.blame = [{id: 'commit id', ranges: [{start: 1, end: 2}]}];
+      element.blame = [
+        {
+          author: 'test-author',
+          time: 12345,
+          commit_msg: '',
+          id: 'commit id',
+          ranges: [{start: 1, end: 2}],
+        },
+      ];
       assert.isTrue(element.classList.contains('showBlame'));
     });
   });
@@ -891,8 +935,10 @@
     const NO_NEWLINE_LEFT = 'No newline at end of left file.';
     const NO_NEWLINE_RIGHT = 'No newline at end of right file.';
 
-    const getWarning = element =>
-      element.shadowRoot.querySelector('.newlineWarning').textContent;
+    const getWarning = (element: GrDiff) => {
+      const warningElement = queryAndAssert(element, '.newlineWarning');
+      return warningElement.textContent;
+    };
 
     setup(() => {
       element = basicFixture.instantiate();
@@ -904,8 +950,9 @@
       element.showNewlineWarningLeft = true;
       element.showNewlineWarningRight = true;
       assert.include(
-          getWarning(element),
-          NO_NEWLINE_LEFT + ' \u2014 ' + NO_NEWLINE_RIGHT);// \u2014 - '—'
+        getWarning(element),
+        NO_NEWLINE_LEFT + ' \u2014 ' + NO_NEWLINE_RIGHT
+      ); // \u2014 - '—'
     });
 
     suite('showNewlineWarningLeft', () => {
@@ -918,11 +965,6 @@
         element.showNewlineWarningLeft = false;
         assert.notInclude(getWarning(element), NO_NEWLINE_LEFT);
       });
-
-      test('hide warning if undefined', () => {
-        element.showNewlineWarningLeft = undefined;
-        assert.notInclude(getWarning(element), NO_NEWLINE_LEFT);
-      });
     });
 
     suite('showNewlineWarningRight', () => {
@@ -935,49 +977,25 @@
         element.showNewlineWarningRight = false;
         assert.notInclude(getWarning(element), NO_NEWLINE_RIGHT);
       });
-
-      test('hide warning if undefined', () => {
-        element.showNewlineWarningRight = undefined;
-        assert.notInclude(getWarning(element), NO_NEWLINE_RIGHT);
-      });
     });
 
     test('_computeNewlineWarningClass', () => {
       const hidden = 'newlineWarning hidden';
       const shown = 'newlineWarning';
-      assert.equal(element._computeNewlineWarningClass(null, true), hidden);
-      assert.equal(element._computeNewlineWarningClass('foo', true), hidden);
-      assert.equal(element._computeNewlineWarningClass(null, false), hidden);
-      assert.equal(element._computeNewlineWarningClass('foo', false), shown);
-    });
-
-    test('_prefsEqual', () => {
-      element = basicFixture.instantiate();
-      assert.isTrue(element._prefsEqual(null, null));
-      assert.isTrue(element._prefsEqual({}, {}));
-      assert.isTrue(element._prefsEqual({x: 1}, {x: 1}));
-      assert.isTrue(
-          element._prefsEqual({x: 1, abc: 'def'}, {x: 1, abc: 'def'}));
-      const somePref = {abc: 'def', p: true};
-      assert.isTrue(element._prefsEqual(somePref, somePref));
-
-      assert.isFalse(element._prefsEqual({}, null));
-      assert.isFalse(element._prefsEqual(null, {}));
-      assert.isFalse(element._prefsEqual({x: 1}, {x: 2}));
-      assert.isFalse(element._prefsEqual({x: 1, y: 'abc'}, {x: 1, y: 'abcd'}));
-      assert.isFalse(element._prefsEqual({x: 1, y: 'abc'}, {x: 1}));
-      assert.isFalse(element._prefsEqual({x: 1}, {x: 1, y: 'abc'}));
+      assert.equal(element._computeNewlineWarningClass(false, true), hidden);
+      assert.equal(element._computeNewlineWarningClass(true, true), hidden);
+      assert.equal(element._computeNewlineWarningClass(false, false), hidden);
+      assert.equal(element._computeNewlineWarningClass(true, false), shown);
     });
   });
 
   suite('key locations', () => {
-    let renderStub;
+    let renderStub: sinon.SinonStub;
 
     setup(() => {
       element = basicFixture.instantiate();
-      element.prefs = {};
-      renderStub = sinon.stub(element.$.diffBuilder, 'render')
-          .returns(new Promise(() => {}));
+      element.prefs = {...MINIMAL_PREFS};
+      renderStub = sinon.stub(element.diffBuilder, 'render');
     });
 
     test('lineOfInterest is a key location', () => {
@@ -994,7 +1012,7 @@
       const threadEl = document.createElement('div');
       threadEl.className = 'comment-thread';
       threadEl.setAttribute('diff-side', 'right');
-      threadEl.setAttribute('line-num', 3);
+      threadEl.setAttribute('line-num', '3');
       element.appendChild(threadEl);
       flush();
 
@@ -1021,7 +1039,11 @@
       });
     });
   });
-  const setupSampleDiff = function(params) {
+  const setupSampleDiff = function (params: {
+    content: DiffContent[];
+    ignore_whitespace?: IgnoreWhitespaceType;
+    binary?: boolean;
+  }) {
     const {ignore_whitespace, content} = params;
     // binary can't be undefined, use false if not set
     const binary = params.binary || false;
@@ -1039,7 +1061,6 @@
       show_whitespace_errors: true,
       syntax_highlighting: true,
       tab_size: 8,
-      theme: 'DEFAULT',
     };
     element.diff = {
       intraline_status: 'OK',
@@ -1059,21 +1080,24 @@
   };
 
   test('clear diff table content as soon as diff changes', () => {
-    const content = [{
-      a: ['all work and no play make andybons a dull boy'],
-    }, {
-      b: [
-        'Non eram nescius, Brute, cum, quae summis ingeniis ',
-      ],
-    }];
+    const content = [
+      {
+        a: ['all work and no play make andybons a dull boy'],
+      },
+      {
+        b: ['Non eram nescius, Brute, cum, quae summis ingeniis '],
+      },
+    ];
     function assertDiffTableWithContent() {
-      assert.isTrue(element.$.diffTable.innerText.includes(content[0].a));
+      const diffTable = element.$.diffTable;
+      assert.isTrue(diffTable.innerText.includes(content[0].a?.[0] ?? ''));
     }
     setupSampleDiff({content});
     assertDiffTableWithContent();
-    element.diff = {...element.diff};
+    element.diff = {...element.diff!};
     // immediately cleaned up
-    assert.equal(element.$.diffTable.innerHTML, '');
+    const diffTable = element.$.diffTable;
+    assert.equal(diffTable.innerHTML, '');
     element._renderDiffTable();
     flush();
     // rendered again
@@ -1082,40 +1106,46 @@
 
   suite('selection test', () => {
     test('user-select set correctly on side-by-side view', () => {
-      const content = [{
-        a: ['all work and no play make andybons a dull boy'],
-        b: ['elgoog elgoog elgoog'],
-      }, {
-        ab: [
-          'Non eram nescius, Brute, cum, quae summis ingeniis ',
-          'exquisitaque doctrina philosophi Graeco sermone tractavissent',
-        ],
-      }];
+      const content = [
+        {
+          a: ['all work and no play make andybons a dull boy'],
+          b: ['elgoog elgoog elgoog'],
+        },
+        {
+          ab: [
+            'Non eram nescius, Brute, cum, quae summis ingeniis ',
+            'exquisitaque doctrina philosophi Graeco sermone tractavissent',
+          ],
+        },
+      ];
       setupSampleDiff({content});
       flush();
-      const diffLine = element.shadowRoot.querySelectorAll('.contentText')[2];
+
+      const diffLine = queryAll<HTMLElement>(element, '.contentText')[2];
       assert.equal(getComputedStyle(diffLine).userSelect, 'none');
-      // click to mark it as selected
-      MockInteractions.tap(diffLine);
+      mouseDown(diffLine);
       assert.equal(getComputedStyle(diffLine).userSelect, 'text');
     });
 
     test('user-select set correctly on unified view', () => {
-      const content = [{
-        a: ['all work and no play make andybons a dull boy'],
-        b: ['elgoog elgoog elgoog'],
-      }, {
-        ab: [
-          'Non eram nescius, Brute, cum, quae summis ingeniis ',
-          'exquisitaque doctrina philosophi Graeco sermone tractavissent',
-        ],
-      }];
+      const content = [
+        {
+          a: ['all work and no play make andybons a dull boy'],
+          b: ['elgoog elgoog elgoog'],
+        },
+        {
+          ab: [
+            'Non eram nescius, Brute, cum, quae summis ingeniis ',
+            'exquisitaque doctrina philosophi Graeco sermone tractavissent',
+          ],
+        },
+      ];
       setupSampleDiff({content});
-      element.viewMode = 'UNIFIED_DIFF';
+      element.viewMode = DiffViewMode.UNIFIED;
       flush();
-      const diffLine = element.shadowRoot.querySelectorAll('.contentText')[2];
+      const diffLine = queryAll<HTMLElement>(element, '.contentText')[2];
       assert.equal(getComputedStyle(diffLine).userSelect, 'none');
-      MockInteractions.tap(diffLine);
+      mouseDown(diffLine);
       assert.equal(getComputedStyle(diffLine).userSelect, 'text');
     });
   });
@@ -1123,71 +1153,87 @@
   suite('whitespace changes only message', () => {
     test('show the message if ignore_whitespace is criteria matches', () => {
       setupSampleDiff({content: [{skip: 100}]});
-      assert.isTrue(element.showNoChangeMessage(
+      assert.isTrue(
+        element.showNoChangeMessage(
           /* loading= */ false,
           element.prefs,
           element._diffLength,
           element.diff
-      ));
+        )
+      );
     });
 
     test('do not show the message for binary files', () => {
       setupSampleDiff({content: [{skip: 100}], binary: true});
-      assert.isFalse(element.showNoChangeMessage(
+      assert.isFalse(
+        element.showNoChangeMessage(
           /* loading= */ false,
           element.prefs,
           element._diffLength,
           element.diff
-      ));
+        )
+      );
     });
 
     test('do not show the message if still loading', () => {
       setupSampleDiff({content: [{skip: 100}]});
-      assert.isFalse(element.showNoChangeMessage(
+      assert.isFalse(
+        element.showNoChangeMessage(
           /* loading= */ true,
           element.prefs,
           element._diffLength,
           element.diff
-      ));
+        )
+      );
     });
 
     test('do not show the message if contains valid changes', () => {
-      const content = [{
-        a: ['all work and no play make andybons a dull boy'],
-        b: ['elgoog elgoog elgoog'],
-      }, {
-        ab: [
-          'Non eram nescius, Brute, cum, quae summis ingeniis ',
-          'exquisitaque doctrina philosophi Graeco sermone tractavissent',
-        ],
-      }];
+      const content = [
+        {
+          a: ['all work and no play make andybons a dull boy'],
+          b: ['elgoog elgoog elgoog'],
+        },
+        {
+          ab: [
+            'Non eram nescius, Brute, cum, quae summis ingeniis ',
+            'exquisitaque doctrina philosophi Graeco sermone tractavissent',
+          ],
+        },
+      ];
       setupSampleDiff({content});
       assert.equal(element._diffLength, 3);
-      assert.isFalse(element.showNoChangeMessage(
+      assert.isFalse(
+        element.showNoChangeMessage(
           /* loading= */ false,
           element.prefs,
           element._diffLength,
           element.diff
-      ));
+        )
+      );
     });
 
     test('do not show message if ignore whitespace is disabled', () => {
-      const content = [{
-        a: ['all work and no play make andybons a dull boy'],
-        b: ['elgoog elgoog elgoog'],
-      }, {
-        ab: [
-          'Non eram nescius, Brute, cum, quae summis ingeniis ',
-          'exquisitaque doctrina philosophi Graeco sermone tractavissent',
-        ],
-      }];
+      const content = [
+        {
+          a: ['all work and no play make andybons a dull boy'],
+          b: ['elgoog elgoog elgoog'],
+        },
+        {
+          ab: [
+            'Non eram nescius, Brute, cum, quae summis ingeniis ',
+            'exquisitaque doctrina philosophi Graeco sermone tractavissent',
+          ],
+        },
+      ];
       setupSampleDiff({ignore_whitespace: 'IGNORE_NONE', content});
-      assert.isFalse(element.showNoChangeMessage(
+      assert.isFalse(
+        element.showNoChangeMessage(
           /* loading= */ false,
           element.prefs,
           element._diffLength,
           element.diff
-      ));
+        )
+      );
     });
   });
 
@@ -1195,21 +1241,4 @@
     const diff = createDiff();
     assert.equal(element.getDiffLength(diff), 52);
   });
-
-  test('_prefsEqual', () => {
-    element = basicFixture.instantiate();
-    assert.isTrue(element._prefsEqual(null, null));
-    assert.isTrue(element._prefsEqual({}, {}));
-    assert.isTrue(element._prefsEqual({x: 1}, {x: 1}));
-    assert.isTrue(element._prefsEqual({x: 1, abc: 'def'}, {x: 1, abc: 'def'}));
-    const somePref = {abc: 'def', p: true};
-    assert.isTrue(element._prefsEqual(somePref, somePref));
-
-    assert.isFalse(element._prefsEqual({}, null));
-    assert.isFalse(element._prefsEqual(null, {}));
-    assert.isFalse(element._prefsEqual({x: 1}, {x: 2}));
-    assert.isFalse(element._prefsEqual({x: 1, y: 'abc'}, {x: 1, y: 'abcd'}));
-    assert.isFalse(element._prefsEqual({x: 1, y: 'abc'}, {x: 1}));
-    assert.isFalse(element._prefsEqual({x: 1}, {x: 1, y: 'abc'}));
-  });
 });
diff --git a/polygerrit-ui/app/embed/gr-diff-app-context-init_test.js b/polygerrit-ui/app/embed/gr-diff-app-context-init_test.js
deleted file mode 100644
index bb46484..0000000
--- a/polygerrit-ui/app/embed/gr-diff-app-context-init_test.js
+++ /dev/null
@@ -1,32 +0,0 @@
-/**
- * @license
- * Copyright (C) 2020 The Android Open Source Project
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- * http://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-
-import '../test/common-test-setup-karma.js';
-import {createDiffAppContext} from './gr-diff-app-context-init.js';
-
-suite('gr diff app context initializer tests', () => {
-  test('all services initialized and are singletons', () => {
-    const appContext = createDiffAppContext();
-    Object.keys(appContext).forEach(serviceName => {
-      const service = appContext[serviceName];
-      assert.isNotNull(service);
-      const service2 = appContext[serviceName];
-      assert.strictEqual(service, service2);
-    });
-  });
-});
-
diff --git a/polygerrit-ui/app/embed/gr-diff-app-context-init_test.ts b/polygerrit-ui/app/embed/gr-diff-app-context-init_test.ts
new file mode 100644
index 0000000..84fd859
--- /dev/null
+++ b/polygerrit-ui/app/embed/gr-diff-app-context-init_test.ts
@@ -0,0 +1,22 @@
+/**
+ * @license
+ * Copyright 2020 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+import {AppContext} from '../services/app-context';
+import '../test/common-test-setup-karma';
+import {createDiffAppContext} from './gr-diff-app-context-init';
+
+suite('gr diff app context initializer tests', () => {
+  test('all services initialized and are singletons', () => {
+    const appContext: AppContext = createDiffAppContext();
+    for (const serviceName of Object.keys(appContext) as Array<
+      keyof AppContext
+    >) {
+      const service = appContext[serviceName];
+      assert.isNotNull(service);
+      const service2 = appContext[serviceName];
+      assert.strictEqual(service, service2);
+    }
+  });
+});
diff --git a/polygerrit-ui/app/services/gr-rest-api/gr-rest-api-impl.ts b/polygerrit-ui/app/services/gr-rest-api/gr-rest-api-impl.ts
index 2b4fc60..3193833 100644
--- a/polygerrit-ui/app/services/gr-rest-api/gr-rest-api-impl.ts
+++ b/polygerrit-ui/app/services/gr-rest-api/gr-rest-api-impl.ts
@@ -2702,20 +2702,22 @@
 
     return Promise.all([promiseA, promiseB]).then(results => {
       // Sometimes the server doesn't send back the content type.
-      const baseImage: Base64ImageFile | null = results[0]
-        ? {
-            ...results[0],
-            _expectedType: diff.meta_a.content_type,
-            _name: diff.meta_a.name,
-          }
-        : null;
-      const revisionImage: Base64ImageFile | null = results[1]
-        ? {
-            ...results[1],
-            _expectedType: diff.meta_b.content_type,
-            _name: diff.meta_b.name,
-          }
-        : null;
+      const baseImage: Base64ImageFile | null =
+        results[0] && diff.meta_a
+          ? {
+              ...results[0],
+              _expectedType: diff.meta_a.content_type,
+              _name: diff.meta_a.name,
+            }
+          : null;
+      const revisionImage: Base64ImageFile | null =
+        results[1] && diff.meta_b
+          ? {
+              ...results[1],
+              _expectedType: diff.meta_b.content_type,
+              _name: diff.meta_b.name,
+            }
+          : null;
       const imagesForDiff: ImagesForDiff = {baseImage, revisionImage};
       return imagesForDiff;
     });
diff --git a/polygerrit-ui/app/test/test-data-generators.ts b/polygerrit-ui/app/test/test-data-generators.ts
index 829a36b..8e8fe42 100644
--- a/polygerrit-ui/app/test/test-data-generators.ts
+++ b/polygerrit-ui/app/test/test-data-generators.ts
@@ -469,6 +469,24 @@
   };
 }
 
+export function createEmptyDiff(): DiffInfo {
+  return {
+    meta_a: {
+      name: 'empty-left.txt',
+      content_type: 'text/plain',
+      lines: 1,
+    },
+    meta_b: {
+      name: 'empty-right.txt',
+      content_type: 'text/plain',
+      lines: 1,
+    },
+    intraline_status: 'OK',
+    change_type: 'MODIFIED',
+    content: [],
+  };
+}
+
 export function createDiff(): DiffInfo {
   return {
     meta_a: {
diff --git a/polygerrit-ui/app/test/test-utils.ts b/polygerrit-ui/app/test/test-utils.ts
index 985bec1..ea7865e 100644
--- a/polygerrit-ui/app/test/test-utils.ts
+++ b/polygerrit-ui/app/test/test-utils.ts
@@ -27,7 +27,7 @@
 import {ShortcutsService} from '../services/shortcuts/shortcuts-service';
 import {queryAndAssert, query} from '../utils/common-util';
 import {FlagsService} from '../services/flags/flags';
-import {Key, Modifier} from '../utils/dom-util';
+import {afterNextRender, Key, Modifier} from '../utils/dom-util';
 import {Observable} from 'rxjs';
 import {filter, take, timeout} from 'rxjs/operators';
 import {HighlightService} from '../services/highlight/highlight-service';
@@ -224,6 +224,10 @@
   return waitUntil(() => stub.called, `${name} was not called`);
 }
 
+export async function nextRender() {
+  return new Promise(resolve => afterNextRender(resolve));
+}
+
 /**
  * Subscribes to the observable and resolves once it emits a matching value.
  * Usage:
diff --git a/polygerrit-ui/app/types/diff.ts b/polygerrit-ui/app/types/diff.ts
index 562d47f..7ad656d 100644
--- a/polygerrit-ui/app/types/diff.ts
+++ b/polygerrit-ui/app/types/diff.ts
@@ -48,9 +48,9 @@
 
 export interface DiffInfo extends DiffInfoApi {
   /** Meta information about the file on side A as a DiffFileMetaInfo entity. */
-  meta_a: DiffFileMetaInfo;
+  meta_a?: DiffFileMetaInfo;
   /** Meta information about the file on side B as a DiffFileMetaInfo entity. */
-  meta_b: DiffFileMetaInfo;
+  meta_b?: DiffFileMetaInfo;
 
   /**
    * Links to the file diff in external sites as a list of DiffWebLinkInfo
diff --git a/polygerrit-ui/app/utils/dom-util.ts b/polygerrit-ui/app/utils/dom-util.ts
index 16e0586..f2e0994 100644
--- a/polygerrit-ui/app/utils/dom-util.ts
+++ b/polygerrit-ui/app/utils/dom-util.ts
@@ -505,3 +505,14 @@
   });
   obs.observe(el);
 }
+
+/**
+ * Mimics a Polymer utility. `requestAnimationFrame` is called before the next
+ * browser paint. An additional `setTimeout` ensures that the paint has
+ * actually happened.
+ */
+export function afterNextRender(callback: (value?: unknown) => void) {
+  requestAnimationFrame(() => {
+    setTimeout(callback);
+  });
+}
diff --git a/polygerrit-ui/app/utils/event-util.ts b/polygerrit-ui/app/utils/event-util.ts
index 418adbd..e624cef 100644
--- a/polygerrit-ui/app/utils/event-util.ts
+++ b/polygerrit-ui/app/utils/event-util.ts
@@ -32,7 +32,7 @@
   );
 }
 
-type HTMLElementEventDetailType<K extends keyof HTMLElementEventMap> =
+export type HTMLElementEventDetailType<K extends keyof HTMLElementEventMap> =
   HTMLElementEventMap[K] extends CustomEvent<infer DT>
     ? unknown extends DT
       ? never