diff --git a/java/com/google/gerrit/server/restapi/change/PostReview.java b/java/com/google/gerrit/server/restapi/change/PostReview.java
index a360510..562bdf8 100644
--- a/java/com/google/gerrit/server/restapi/change/PostReview.java
+++ b/java/com/google/gerrit/server/restapi/change/PostReview.java
@@ -1272,28 +1272,27 @@
             del.add(c);
             update.putApproval(normName, (short) 0);
           }
-        } else if (c != null) {
-          // Check if the label exists in the request input (the user voted again). If the user
-          // hadn't voted again, there is no need to re-apply the vote.
-          if (inLabels.keySet().contains(c.label())) {
-            PatchSetApproval.Builder b =
-                c.toBuilder()
-                    .value(ent.getValue())
-                    .granted(ctx.getWhen())
-                    .tag(Optional.ofNullable(in.tag));
-            ctx.getUser().updateRealAccountId(b::realAccountId);
-            c = b.build();
-            ups.add(c);
-            addLabelDelta(normName, c.value());
-            oldApprovals.put(normName, previous.get(normName));
-            approvals.put(normName, c.value());
-            update.putApproval(normName, ent.getValue());
-          } else {
-            current.put(normName, c);
-            oldApprovals.put(normName, null);
-            approvals.put(normName, c.value());
-          }
-        } else {
+          // Only allow voting again if the vote is copied over from a past patch-set, or the
+          // values are different.
+        } else if (c != null
+            && (c.value() != ent.getValue() || isApprovalCopiedOver(c, ctx.getNotes()))) {
+          PatchSetApproval.Builder b =
+              c.toBuilder()
+                  .value(ent.getValue())
+                  .granted(ctx.getWhen())
+                  .tag(Optional.ofNullable(in.tag));
+          ctx.getUser().updateRealAccountId(b::realAccountId);
+          c = b.build();
+          ups.add(c);
+          addLabelDelta(normName, c.value());
+          oldApprovals.put(normName, previous.get(normName));
+          approvals.put(normName, c.value());
+          update.putApproval(normName, ent.getValue());
+        } else if (c != null && c.value() == ent.getValue()) {
+          current.put(normName, c);
+          oldApprovals.put(normName, null);
+          approvals.put(normName, c.value());
+        } else if (c == null) {
           c =
               ApprovalsUtil.newApproval(psId, user, lt.getLabelId(), ent.getValue(), ctx.getWhen())
                   .tag(Optional.ofNullable(in.tag))
@@ -1319,6 +1318,17 @@
       return !del.isEmpty() || !ups.isEmpty();
     }
 
+    /**
+     * Approval is copied over if it doesn't exist in the approvals of the current patch-set
+     * according to change notes (which means it was computed in {@link
+     * com.google.gerrit.server.ApprovalInference})
+     */
+    private boolean isApprovalCopiedOver(
+        PatchSetApproval patchSetApproval, ChangeNotes changeNotes) {
+      return !changeNotes.getApprovals().get(changeNotes.getChange().currentPatchSetId()).stream()
+          .anyMatch(p -> p.equals(patchSetApproval));
+    }
+
     private void validatePostSubmitLabels(
         ChangeContext ctx,
         LabelTypes labelTypes,
diff --git a/javatests/com/google/gerrit/acceptance/api/change/PostReviewIT.java b/javatests/com/google/gerrit/acceptance/api/change/PostReviewIT.java
index 0b2e0f7..c8b1715 100644
--- a/javatests/com/google/gerrit/acceptance/api/change/PostReviewIT.java
+++ b/javatests/com/google/gerrit/acceptance/api/change/PostReviewIT.java
@@ -63,7 +63,6 @@
 import com.google.gerrit.server.restapi.change.OnPostReview;
 import com.google.gerrit.server.restapi.change.PostReview;
 import com.google.gerrit.server.update.CommentsRejectedException;
-import com.google.gerrit.testing.FakeEmailSender;
 import com.google.gerrit.testing.TestCommentHelper;
 import com.google.inject.Inject;
 import com.google.inject.Module;
@@ -597,7 +596,7 @@
       input = new ReviewInput().label(LabelId.CODE_REVIEW, 2);
       gApi.changes().id(r.getChangeId()).current().review(input);
       testOnPostReview.assertApproval(
-          LabelId.CODE_REVIEW, /* expectedOldValue= */ 2, /* expectedNewValue= */ 2);
+          LabelId.CODE_REVIEW, /* expectedOldValue= */ null, /* expectedNewValue= */ 2);
 
       // Delete the vote.
       input = new ReviewInput().label(LabelId.CODE_REVIEW, 0);
@@ -627,21 +626,19 @@
     assertThat(r.getChange().approvals().values()).hasSize(1);
     List<ChangeMessageInfo> changeMessages = gApi.changes().id(r.getChangeId()).messages();
 
-    // The two latest change messages are both about Code-Review+2
+    // Only the last change message is about Code-Review+2
     assertThat(Iterables.getLast(changeMessages).message).isEqualTo("Patch Set 1: Code-Review+2");
     changeMessages.remove(changeMessages.size() - 1);
-    assertThat(Iterables.getLast(changeMessages).message).isEqualTo("Patch Set 1: Code-Review+2");
+    assertThat(Iterables.getLast(changeMessages).message)
+        .isNotEqualTo("Patch Set 1: Code-Review+2");
 
-    // The two latest emails are about Code-Review +2.
-    List<FakeEmailSender.Message> messages = sender.getMessages();
-    assertThat(messages).hasSize(2);
-    for (FakeEmailSender.Message message : messages) {
-      assertThat(message.body()).contains("Patch Set 1: Code-Review+2");
-    }
+    // Only one email is about Code-Review +2 was sent.
+    assertThat(Iterables.getOnlyElement(sender.getMessages()).body())
+        .contains("Patch Set 1: Code-Review+2");
   }
 
   @Test
-  public void votingTheSameVoteSecondTimeExtendsOnPostReview() throws Exception {
+  public void votingTheSameVoteSecondTimeExtendsOnPostReviewWithOldNullValue() throws Exception {
     PushOneCommit.Result r = createChange();
 
     // Add a new vote.
@@ -656,12 +653,12 @@
       gApi.changes().id(r.getChangeId()).current().review(input);
 
       testOnPostReview.assertApproval(
-          LabelId.CODE_REVIEW, /* expectedOldValue= */ 2, /* expectedNewValue= */ 2);
+          LabelId.CODE_REVIEW, /* expectedOldValue= */ null, /* expectedNewValue= */ 2);
     }
   }
 
   @Test
-  public void votingTheSameVoteSecondTimeFiresOnCommentAdded() throws Exception {
+  public void votingTheSameVoteSecondTimeDoesNotFireOnCommentAdded() throws Exception {
     PushOneCommit.Result r = createChange();
 
     // Add a new vote.
@@ -675,8 +672,8 @@
       input = new ReviewInput().label(LabelId.CODE_REVIEW, 2);
       gApi.changes().id(r.getChangeId()).current().review(input);
 
-      assertThat(testListener.lastCommentAddedEvent.getComment())
-          .isEqualTo("Patch Set 1: Code-Review+2");
+      // Event not fired.
+      assertThat(testListener.lastCommentAddedEvent).isNull();
     }
   }
 
diff --git a/javatests/com/google/gerrit/acceptance/api/revision/RevisionIT.java b/javatests/com/google/gerrit/acceptance/api/revision/RevisionIT.java
index 67e62dd..abfd7896 100644
--- a/javatests/com/google/gerrit/acceptance/api/revision/RevisionIT.java
+++ b/javatests/com/google/gerrit/acceptance/api/revision/RevisionIT.java
@@ -176,12 +176,12 @@
     assertThat(approval.postSubmit).isNull();
     assertPermitted(gApi.changes().id(changeId).get(DETAILED_LABELS), LabelId.CODE_REVIEW, 1, 2);
 
-    // Repeating the current label is allowed. Flips the postSubmit since technically this is a
-    // new vote.
+    // Repeating the current label is allowed. Does not flip the postSubmit bit due to
+    // deduplication codepath.
     gApi.changes().id(changeId).current().review(ReviewInput.recommend());
     approval = getApproval(changeId, label);
     assertThat(approval.value).isEqualTo(1);
-    assertThat(approval.postSubmit).isTrue();
+    assertThat(approval.postSubmit).isNull();
 
     // Reducing vote is not allowed.
     ResourceConflictException thrown =
@@ -193,7 +193,7 @@
         .isEqualTo("Cannot reduce vote on labels for closed change: Code-Review");
     approval = getApproval(changeId, label);
     assertThat(approval.value).isEqualTo(1);
-    assertThat(approval.postSubmit).isTrue();
+    assertThat(approval.postSubmit).isNull();
 
     // Increasing vote is allowed.
     gApi.changes().id(changeId).current().review(ReviewInput.approve());
diff --git a/polygerrit-ui/app/api/annotation.ts b/polygerrit-ui/app/api/annotation.ts
index c046b4f..e58bdd5 100644
--- a/polygerrit-ui/app/api/annotation.ts
+++ b/polygerrit-ui/app/api/annotation.ts
@@ -14,18 +14,14 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-import {CoverageRange, Side} from './diff';
+import {CoverageRange, GrDiffLine, Side} from './diff';
 import {StyleObject} from './styles';
 
-export type AddLayerFunc = (ctx: AnnotationContext) => void;
-
-export type NotifyFunc = (
-  path: string,
-  start: number,
-  end: number,
-  side: Side
-) => void;
-
+/**
+ * This is the callback object that Gerrit calls once for each diff. Gerrit
+ * is then responsible for styling the diff according the returned array of
+ * CoverageRanges.
+ */
 export type CoverageProvider = (
   changeNum: number,
   path: string,
@@ -34,14 +30,35 @@
   /**
    * This is a ChangeInfo object as defined here:
    * https://gerrit-review.googlesource.com/Documentation/rest-api-changes.html#change-info
-   * We neither want to repeat it nor add a dependency on it here.
+   * At the moment we neither want to repeat it nor add a dependency on it here.
+   * TODO: Create a dedicated smaller object for exposing a change in the plugin
+   * API. Or allow the plugin API to depend on the entire rest API.
    */
   change?: unknown
 ) => Promise<Array<CoverageRange>>;
 
+export type AnnotationCallback = (ctx: AnnotationContext) => void;
+
+/**
+ * This object is passed to the plugin from Gerrit for each line of a diff that
+ * is being rendered. The plugin can then call annotateRange() or
+ * annotateLineNumber() to apply additional styles to the diff.
+ */
 export interface AnnotationContext {
+  /** Set by Gerrit and consumed by the plugin provided AddLayerFunc. */
+  readonly changeNum: number;
+  /** Set by Gerrit and consumed by the plugin provided AddLayerFunc. */
+  readonly path: string;
+  /** Set by Gerrit and consumed by the plugin provided AddLayerFunc. */
+  readonly line: GrDiffLine;
+  /** Set by Gerrit and consumed by the plugin provided AddLayerFunc. */
+  readonly contentEl: HTMLElement;
+  /** Set by Gerrit and consumed by the plugin provided AddLayerFunc. */
+  readonly lineNumberEl: HTMLElement;
+
   /**
-   * Method to add annotations to a content line.
+   * Can be called by the plugin to style a part of the given line of the
+   * context.
    *
    * @param offset The char offset where the update starts.
    * @param length The number of chars that the update covers.
@@ -56,7 +73,8 @@
   ): void;
 
   /**
-   * Method to add a CSS class to the line number TD element.
+   * Can be called by the plugin to style a part of the given line of the
+   * context.
    *
    * @param styleObject The style object for the range.
    * @param side The side of the update. ('left' or 'right')
@@ -66,23 +84,12 @@
 
 export interface AnnotationPluginApi {
   /**
-   * Register a function to call to apply annotations. Plugins should use
-   * GrAnnotationActionsContext.annotateRange and
-   * GrAnnotationActionsContext.annotateLineNumber to apply a CSS class to the
-   * line content or the line number.
-   *
-   * @param addLayerFunc The function
-   * that will be called when the AnnotationLayer is ready to annotate.
+   * Registers a callback for applying annotations. Gerrit will call the
+   * callback for every line of every file that is rendered and pass the
+   * information about the file and line as an AnnotationContext, which also
+   * provides methods for the plugin to style the content.
    */
-  addLayer(addLayerFunc: AddLayerFunc): AnnotationPluginApi;
-
-  /**
-   * The specified function will be called with a notify function for the plugin
-   * to call when it has all required data for annotation. Optional.
-   *
-   * @param notifyFunc See doc of the notify function below to see what it does.
-   */
-  addNotifier(notifyFunc: (n: NotifyFunc) => void): AnnotationPluginApi;
+  setLayer(callback: AnnotationCallback): AnnotationPluginApi;
 
   /**
    * The specified function will be called when a gr-diff component is built,
@@ -117,9 +124,10 @@
   ): AnnotationPluginApi;
 
   /**
-   * The notify function will call the listeners of all required annotation
-   * layers. Intended to be called by the plugin when all required data for
-   * annotation is available.
+   * For plugins notifying Gerrit about new annotations being ready to be
+   * applied for a certain range. Gerrit will then re-render the relevant lines
+   * of the diff and call back to the layer annotation function that was
+   * registered in addLayer().
    *
    * @param path The file path whose listeners should be notified.
    * @param start The line where the update starts.
diff --git a/polygerrit-ui/app/elements/change/gr-change-summary/gr-change-summary.ts b/polygerrit-ui/app/elements/change/gr-change-summary/gr-change-summary.ts
index 72ed6e6..4af51a9 100644
--- a/polygerrit-ui/app/elements/change/gr-change-summary/gr-change-summary.ts
+++ b/polygerrit-ui/app/elements/change/gr-change-summary/gr-change-summary.ts
@@ -40,6 +40,7 @@
   iconForCategory,
   iconForStatus,
   isRunning,
+  isRunningOrHasCompleted,
 } from '../../../services/checks/checks-util';
 import {ChangeComments} from '../../diff/gr-comment-api/gr-comment-api';
 import {
@@ -288,6 +289,9 @@
         :host.new-change-summary-true {
           margin-bottom: var(--spacing-m);
         }
+        .zeroState {
+          color: var(--primary-text-color);
+        }
         td.key {
           padding-right: var(--spacing-l);
           padding-bottom: var(--spacing-m);
@@ -312,6 +316,11 @@
     ];
   }
 
+  renderChecksZeroState() {
+    if (this.runs.some(isRunningOrHasCompleted)) return;
+    return html`<span class="font-small zeroState">No results</span>`;
+  }
+
   renderChecksChipForCategory(category: Category) {
     const icon = iconForCategory(category);
     const runs = this.runs.filter(run => hasResultsOf(run, category));
@@ -395,7 +404,7 @@
           <tr ?hidden=${!this.showChecksSummary}>
             <td class="key">Checks</td>
             <td class="value">
-              ${this.renderChecksChipForCategory(
+              ${this.renderChecksZeroState()}${this.renderChecksChipForCategory(
                 Category.ERROR
               )}${this.renderChecksChipForCategory(
                 Category.WARNING
@@ -410,13 +419,13 @@
           <tr ?hidden=${!this.newChangeSummaryUiEnabled}>
             <td class="key">Comments</td>
             <td class="value">
-              <gr-summary-chip
-                styleType=${SummaryChipStyles.INFO}
+              <span
+                class="font-small zeroState"
                 ?hidden=${!!countResolvedComments ||
                 !!draftCount ||
                 !!countUnresolvedComments}
               >
-                No Comments</gr-summary-chip
+                No Comments</span
               ><gr-summary-chip
                 styleType=${SummaryChipStyles.WARNING}
                 category=${CommentTabState.DRAFTS}
diff --git a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.ts b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.ts
index c33eb26..8f74f76 100644
--- a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.ts
+++ b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view.ts
@@ -118,7 +118,7 @@
   GrCommentApi,
   ChangeComments,
 } from '../../diff/gr-comment-api/gr-comment-api';
-import {hasOwnProperty} from '../../../utils/common-util';
+import {assertIsDefined, hasOwnProperty} from '../../../utils/common-util';
 import {GrEditControls} from '../../edit/gr-edit-controls/gr-edit-controls';
 import {
   CommentThread,
@@ -883,7 +883,7 @@
   }
 
   _handleCommitMessageSave(e: EditableContentSaveEvent) {
-    if (!this._change) throw new Error('missing required change property');
+    assertIsDefined(this._change, '_change');
     if (!this._changeNum)
       throw new Error('missing required changeNum property');
     // Trim trailing whitespace from each line.
@@ -1424,7 +1424,7 @@
   }
 
   _handleMessageAnchorTap(e: CustomEvent<{id: string}>) {
-    if (!this._change) throw new Error('missing required change property');
+    assertIsDefined(this._change, '_change');
     if (!this._patchRange)
       throw new Error('missing required _patchRange property');
     const hash = MSG_PREFIX + e.detail.id;
@@ -1706,7 +1706,7 @@
     if (this.shouldSuppressKeyboardShortcut(e)) {
       return;
     }
-    if (!this._change) throw new Error('missing required change property');
+    assertIsDefined(this._change, '_change');
     if (!this._patchRange)
       throw new Error('missing required _patchRange property');
     if (this._patchRange.basePatchNum === ParentPatchSetNum) {
@@ -1720,7 +1720,7 @@
     if (this.shouldSuppressKeyboardShortcut(e)) {
       return;
     }
-    if (!this._change) throw new Error('missing required change property');
+    assertIsDefined(this._change, '_change');
     if (!this._patchRange)
       throw new Error('missing required _patchRange property');
     if (this._patchRange.basePatchNum === ParentPatchSetNum) {
@@ -1734,7 +1734,7 @@
     if (this.shouldSuppressKeyboardShortcut(e)) {
       return;
     }
-    if (!this._change) throw new Error('missing required change property');
+    assertIsDefined(this._change, '_change');
     if (!this._patchRange)
       throw new Error('missing required _patchRange property');
     const latestPatchNum = computeLatestPatchNum(this._allPatchSets);
@@ -1753,7 +1753,7 @@
     if (this.shouldSuppressKeyboardShortcut(e)) {
       return;
     }
-    if (!this._change) throw new Error('missing required change property');
+    assertIsDefined(this._change, '_change');
     const latestPatchNum = computeLatestPatchNum(this._allPatchSets);
     if (!this._patchRange)
       throw new Error('missing required _patchRange property');
@@ -1772,7 +1772,7 @@
     if (this.shouldSuppressKeyboardShortcut(e)) {
       return;
     }
-    if (!this._change) throw new Error('missing required change property');
+    assertIsDefined(this._change, '_change');
     if (!this._patchRange)
       throw new Error('missing required _patchRange property');
     const latestPatchNum = computeLatestPatchNum(this._allPatchSets);
@@ -1918,7 +1918,7 @@
   }
 
   _getProjectConfig() {
-    if (!this._change) throw new Error('missing required change property');
+    assertIsDefined(this._change, '_change');
     return this.restApiService
       .getProjectConfig(this._change.project)
       .then(config => {
@@ -2314,7 +2314,7 @@
    * (`this._patchRange`) being defined.
    */
   _reloadPatchNumDependentResources(rightPatchNumChanged?: boolean) {
-    if (!this._changeNum) throw new Error('missing changeNum');
+    assertIsDefined(this._changeNum, '_changeNum');
     if (!this._patchRange?.patchNum) throw new Error('missing patchNum');
     const promises = [this._getCommitInfo(), this.$.fileList.reload()];
     if (rightPatchNumChanged)
@@ -2554,7 +2554,7 @@
     }
 
     this._updateCheckTimerHandle = this.async(() => {
-      if (!this._change) throw new Error('missing required change property');
+      assertIsDefined(this._change, '_change');
       const change = this._change;
       fetchChangeUpdates(change, this.restApiService).then(result => {
         let toastMessage = null;
@@ -2662,7 +2662,7 @@
       GrEditControls
     >('#editControls');
     if (!controls) throw new Error('Missing edit controls');
-    if (!this._change) throw new Error('missing required change property');
+    assertIsDefined(this._change, '_change');
     if (!this._patchRange)
       throw new Error('missing required _patchRange property');
     const path = e.detail.path;
@@ -2697,7 +2697,7 @@
     if (!this._selectedRevision) {
       return;
     }
-    if (!this._change) throw new Error('missing required change property');
+    assertIsDefined(this._change, '_change');
 
     let patchNum: PatchSetNum;
     if (patchNumStr === 'edit') {
@@ -2745,7 +2745,7 @@
   }
 
   _handleStopEditTap() {
-    if (!this._change) throw new Error('missing required change property');
+    assertIsDefined(this._change, '_change');
     if (!this._patchRange)
       throw new Error('missing required _patchRange property');
     GerritNav.navigateToChange(this._change, this._patchRange.patchNum);
diff --git a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view_html.ts b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view_html.ts
index 56c1d38..f3fb860 100644
--- a/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view_html.ts
+++ b/polygerrit-ui/app/elements/change/gr-change-view/gr-change-view_html.ts
@@ -105,7 +105,7 @@
       font-size: var(--font-size-mono);
       line-height: var(--line-height-mono);
       margin-right: var(--spacing-l);
-      margin-bottom: var(--spacing-s);
+      margin-bottom: var(--spacing-m);
       /* Account for border and padding and rounding errors. */
       max-width: calc(72ch + 2px + 2 * var(--spacing-m) + 0.4px);
     }
diff --git a/polygerrit-ui/app/elements/change/gr-related-changes-list/gr-related-changes-list.ts b/polygerrit-ui/app/elements/change/gr-related-changes-list/gr-related-changes-list.ts
index b4b6d31..5705c4f 100644
--- a/polygerrit-ui/app/elements/change/gr-related-changes-list/gr-related-changes-list.ts
+++ b/polygerrit-ui/app/elements/change/gr-related-changes-list/gr-related-changes-list.ts
@@ -451,12 +451,14 @@
       ?.getElementsByClassName('arrowToCurrentChange')[0]
       ?.nextElementSibling?.nextElementSibling?.getElementsByTagName('a')[0];
 
-    if (!target || !currentChange) return;
+    if (!target) return;
     this.reportingService.reportInteraction('related-change-click', {
       sectionName,
       index: sectionLinks.indexOf(target) + 1,
       countChanges: sectionLinks.length,
-      currentChangeIndex: sectionLinks.indexOf(currentChange) + 1,
+      currentChangeIndex: !currentChange
+        ? undefined
+        : sectionLinks.indexOf(currentChange) + 1,
     });
   }
 }
diff --git a/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.ts b/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.ts
index 36764f3..705a402 100644
--- a/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.ts
+++ b/polygerrit-ui/app/elements/change/gr-reply-dialog/gr-reply-dialog.ts
@@ -92,6 +92,7 @@
 } from '@polymer/polymer/interfaces';
 import {
   areSetsEqual,
+  assertIsDefined,
   assertNever,
   containsAll,
 } from '../../../utils/common-util';
@@ -433,7 +434,7 @@
   }
 
   open(focusTarget?: FocusTarget) {
-    if (!this.change) throw new Error('missing required change property');
+    assertIsDefined(this.change, 'change');
     this.knownLatestState = LatestPatchState.CHECKING;
     fetchChangeUpdates(this.change, this.restApiService).then(result => {
       this.knownLatestState = result.isLatest
@@ -605,7 +606,7 @@
     account: AccountInfoInput | GroupInfoInput,
     type: ReviewerType
   ) {
-    if (!this.change) throw new Error('missing required change property');
+    assertIsDefined(this.change, 'change');
     if (account._pendingAdd || !isAccount(account)) {
       return;
     }
@@ -1213,7 +1214,7 @@
   }
 
   cancel() {
-    if (!this.change) throw new Error('missing required change property');
+    assertIsDefined(this.change, 'change');
     if (!this._owner) throw new Error('missing required _owner property');
     this.dispatchEvent(
       new CustomEvent('cancel', {
@@ -1269,8 +1270,8 @@
   }
 
   _saveReview(review: ReviewInput, errFn?: ErrorCallback) {
-    if (!this.change) throw new Error('missing required change property');
-    if (!this.patchNum) throw new Error('missing required patchNum property');
+    assertIsDefined(this.change, 'change');
+    assertIsDefined(this.patchNum, 'patchNum');
     return this.restApiService.saveChangeReview(
       this.change._number,
       this.patchNum,
@@ -1316,7 +1317,7 @@
   }
 
   _getStorageLocation(): StorageLocation {
-    if (!this.change) throw new Error('missing required change property');
+    assertIsDefined(this.change, 'change');
     return {
       changeNum: this.change._number,
       patchNum: '@change',
diff --git a/polygerrit-ui/app/elements/checks/gr-checks-results.ts b/polygerrit-ui/app/elements/checks/gr-checks-results.ts
index bc72b2a..df881f3 100644
--- a/polygerrit-ui/app/elements/checks/gr-checks-results.ts
+++ b/polygerrit-ui/app/elements/checks/gr-checks-results.ts
@@ -21,8 +21,10 @@
 import {sharedStyles} from '../../styles/shared-styles';
 import {RunResult} from '../../services/checks/checks-model';
 import {
+  hasCompleted,
   hasCompletedWithoutResults,
   iconForCategory,
+  isRunning,
 } from '../../services/checks/checks-util';
 
 @customElement('gr-result-row')
@@ -276,6 +278,9 @@
         .categoryHeader iron-icon.success {
           color: var(--success-foreground);
         }
+        .noCompleted {
+          margin-top: var(--spacing-l);
+        }
         table.resultsTable {
           width: 100%;
           max-width: 1280px;
@@ -295,12 +300,21 @@
   render() {
     return html`
       <div><h2 class="heading-2">Results</h2></div>
-      ${this.renderSection(Category.ERROR)}
+      ${this.renderNoCompleted()} ${this.renderSection(Category.ERROR)}
       ${this.renderSection(Category.WARNING)}
       ${this.renderSection(Category.INFO)} ${this.renderSuccess()}
     `;
   }
 
+  renderNoCompleted() {
+    if (this.runs.some(hasCompleted)) return;
+    let text = 'No results';
+    if (this.runs.some(isRunning)) {
+      text = 'Checks are running ...';
+    }
+    return html`<div class="noCompleted">${text}</div>`;
+  }
+
   renderSection(category: Category) {
     const catString = category.toString().toLowerCase();
     const runs = this.runs.filter(r =>
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.ts b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.ts
index ff60531..47b4a1f 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.ts
+++ b/polygerrit-ui/app/elements/diff/gr-diff-host/gr-diff-host.ts
@@ -79,6 +79,7 @@
   fireEvent,
 } from '../../../utils/event-util';
 import {getPluginLoader} from '../../shared/gr-js-api-interface/gr-plugin-loader';
+import {assertIsDefined} from '../../../utils/common-util';
 
 const MSG_EMPTY_BLAME = 'No blame information for this diff.';
 
@@ -324,8 +325,8 @@
     return getPluginLoader()
       .awaitPluginsLoaded()
       .then(() => {
-        if (!this.path) throw new Error('Missing required "path" property.');
-        if (!this.changeNum) throw new Error('Missing required "changeNum".');
+        assertIsDefined(this.path, 'path');
+        assertIsDefined(this.changeNum, 'changeNum');
         this._layers = this._getLayers(this.path, this.changeNum);
         this._coverageRanges = [];
         // We kick off fetching the data here, but we don't return the promise,
@@ -341,8 +342,8 @@
    */
   async reload(shouldReportMetric?: boolean) {
     this.clear();
-    if (!this.path) throw new Error('Missing required "path" property.');
-    if (!this.changeNum) throw new Error('Missing required "changeNum" prop.');
+    assertIsDefined(this.path, 'path');
+    assertIsDefined(this.changeNum, 'changeNum');
     this.diff = undefined;
     this._errorMessage = null;
     const whitespaceLevel = this._getIgnoreWhitespace();
@@ -420,10 +421,10 @@
   }
 
   _getCoverageData() {
-    if (!this.changeNum) throw new Error('Missing required "changeNum" prop.');
-    if (!this.change) throw new Error('Missing required "change" prop.');
-    if (!this.path) throw new Error('Missing required "path" prop.');
-    if (!this.patchRange) throw new Error('Missing required "patchRange".');
+    assertIsDefined(this.changeNum, 'changeNum');
+    assertIsDefined(this.change, 'change');
+    assertIsDefined(this.path, 'path');
+    assertIsDefined(this.patchRange, 'patchRange');
     const changeNum = this.changeNum;
     const change = this.change;
     const path = this.path;
@@ -442,7 +443,7 @@
           if (!provider) return;
           provider(changeNum, path, basePatchNum, patchNum, change)
             .then(coverageRanges => {
-              if (!this.patchRange) throw new Error('Missing "patchRange".');
+              assertIsDefined(this.patchRange, 'patchRange');
               if (
                 !coverageRanges ||
                 changeNum !== this.changeNum ||
@@ -532,9 +533,9 @@
    * Load and display blame information for the base of the diff.
    */
   loadBlame(): Promise<BlameInfo[]> {
-    if (!this.changeNum) throw new Error('Missing required "changeNum" prop.');
-    if (!this.patchRange) throw new Error('Missing required "patchRange".');
-    if (!this.path) throw new Error('Missing required "path" property.');
+    assertIsDefined(this.changeNum, 'changeNum');
+    assertIsDefined(this.patchRange, 'patchRange');
+    assertIsDefined(this.path, 'path');
     return this.restApiService
       .getBlame(this.changeNum, this.patchRange.patchNum, this.path, true)
       .then(blame => {
@@ -599,9 +600,9 @@
     // Wrap the diff request in a new promise so that the error handler
     // rejects the promise, allowing the error to be handled in the .catch.
     return new Promise((resolve, reject) => {
-      if (!this.changeNum) throw new Error('Missing required "changeNum".');
-      if (!this.patchRange) throw new Error('Missing required "patchRange".');
-      if (!this.path) throw new Error('Missing required "path" property.');
+      assertIsDefined(this.changeNum, 'changeNum');
+      assertIsDefined(this.patchRange, 'patchRange');
+      assertIsDefined(this.path, 'path');
       this.restApiService
         .getDiff(
           this.changeNum,
@@ -669,7 +670,7 @@
 
     // Report the due_to_rebase percentage in the "diff" category when
     // applicable.
-    if (!this.patchRange) throw new Error('Missing required "patchRange".');
+    assertIsDefined(this.patchRange, 'patchRange');
     if (this.patchRange.basePatchNum === 'PARENT') {
       this.reporting.reportInteraction(EVENT_AGAINST_PARENT);
     } else if (percentRebaseDelta === 0) {
@@ -726,8 +727,8 @@
   }
 
   _getImages(diff: DiffInfo) {
-    if (!this.changeNum) throw new Error('Missing required "changeNum" prop.');
-    if (!this.patchRange) throw new Error('Missing required "patchRange".');
+    assertIsDefined(this.changeNum, 'changeNum');
+    assertIsDefined(this.patchRange, 'patchRange');
     return this.restApiService.getImagesForDiff(
       this.changeNum,
       diff,
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.ts b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.ts
index b31333c..088d9cf 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.ts
+++ b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view.ts
@@ -97,6 +97,7 @@
 import {CustomKeyboardEvent, OpenFixPreviewEvent} from '../../../types/events';
 import {fireAlert, fireTitleChange} from '../../../utils/event-util';
 import {GerritView} from '../../../services/router/router-model';
+import {assertIsDefined} from '../../../utils/common-util';
 const ERR_REVIEW_STATUS = 'Couldn’t change file review status.';
 const MSG_LOADING_BLAME = 'Loading blame...';
 const MSG_LOADED_BLAME = 'Blame loaded';
@@ -370,7 +371,7 @@
   }
 
   _getChangeEdit() {
-    if (!this._changeNum) throw new Error('Missing this._changeNum');
+    assertIsDefined(this._changeNum, '_changeNum');
     return this.restApiService.getChangeEdit(this._changeNum);
   }
 
@@ -980,7 +981,7 @@
         leftSide = !!this.params.leftSide;
       }
     }
-    if (!this._patchRange) throw new Error('Failed to initialize patchRange.');
+    assertIsDefined(this._patchRange, '_patchRange');
     this._initLineOfInterestAndCursor(leftSide);
 
     if (this.params?.commentId) {
@@ -1052,10 +1053,10 @@
         this._initPatchRange();
         this._initCommitRange();
 
-        if (!this._path) throw new Error('path must be defined');
+        assertIsDefined(this._path, '_path');
         if (!this._changeComments)
           throw new Error('change comments must be defined');
-        if (!this._patchRange) throw new Error('patch range must be defined');
+        assertIsDefined(this._patchRange, '_patchRange');
 
         // TODO(dhruvsri): check if basePath should be set here
         this.$.diffHost.threads = this._changeComments.getThreadsBySideForFile(
@@ -1082,9 +1083,9 @@
         if (!this._diff) throw new Error('Missing this._diff');
         const fileUnchanged = this._isFileUnchanged(this._diff);
         if (fileUnchanged && value.commentLink) {
-          if (!this._change) throw new Error('Missing this._change');
-          if (!this._path) throw new Error('Missing this._path');
-          if (!this._patchRange) throw new Error('Missing this._patchRange');
+          assertIsDefined(this._change, '_change');
+          assertIsDefined(this._path, '_path');
+          assertIsDefined(this._patchRange, '_patchRange');
 
           if (this._patchRange.basePatchNum === ParentPatchSetNum) {
             // file is unchanged between Base vs X
@@ -1493,7 +1494,7 @@
   }
 
   _loadComments(patchSet?: PatchSetNum) {
-    if (!this._changeNum) throw new Error('Missing this._changeNum');
+    assertIsDefined(this._changeNum, '_changeNum');
     return this.$.commentAPI
       .loadAll(this._changeNum, patchSet)
       .then(comments => {
@@ -1529,7 +1530,7 @@
   }
 
   _getDiffDrafts() {
-    if (!this._changeNum) throw new Error('Missing this._changeNum');
+    assertIsDefined(this._changeNum, '_changeNum');
 
     return this.restApiService.getDiffDrafts(this._changeNum);
   }
diff --git a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.ts b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.ts
index 189bac7..6974a76 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.ts
+++ b/polygerrit-ui/app/elements/diff/gr-diff/gr-diff.ts
@@ -75,6 +75,7 @@
   RenderPreferences,
 } from '../../../api/diff';
 import {isSafari} from '../../../utils/dom-util';
+import {assertIsDefined} from '../../../utils/common-util';
 
 const NO_NEWLINE_BASE = 'No newline at end of base file.';
 const NO_NEWLINE_REVISION = 'No newline at end of revision file.';
@@ -629,7 +630,7 @@
     const contentEl = this.$.diffBuilder.getContentTdByLineEl(lineEl);
     if (!contentEl) throw new Error('content el not found for line el');
     side = side ?? this._getCommentSideByLineAndContent(lineEl, contentEl);
-    if (!this.path) throw new Error('must have a path to create comments');
+    assertIsDefined(this.path, 'path');
     this.dispatchEvent(
       new CustomEvent<CreateCommentEventDetail>('create-comment', {
         bubbles: true,
diff --git a/polygerrit-ui/app/elements/edit/gr-editor-view/gr-editor-view.ts b/polygerrit-ui/app/elements/edit/gr-editor-view/gr-editor-view.ts
index f6f4395..1864598 100644
--- a/polygerrit-ui/app/elements/edit/gr-editor-view/gr-editor-view.ts
+++ b/polygerrit-ui/app/elements/edit/gr-editor-view/gr-editor-view.ts
@@ -45,6 +45,7 @@
 import {fireAlert, fireTitleChange} from '../../../utils/event-util';
 import {appContext} from '../../../services/app-context';
 import {ErrorCallback} from '../../../api/rest';
+import {assertIsDefined} from '../../../utils/common-util';
 
 const RESTORED_MESSAGE = 'Content restored from a previous edit.';
 const SAVING_MESSAGE = 'Saving changes...';
@@ -326,7 +327,7 @@
   }
 
   _handlePublishTap() {
-    if (!this._changeNum) throw new Error('missing changeNum');
+    assertIsDefined(this._changeNum, '_changeNum');
 
     const changeNum = this._changeNum;
     this._saveEdit().then(() => {
@@ -347,7 +348,7 @@
           handleError
         )
         .then(() => {
-          if (!this._change) throw new Error('missing change');
+          assertIsDefined(this._change, '_change');
           GerritNav.navigateToChange(this._change);
         });
     });
diff --git a/polygerrit-ui/app/elements/shared/gr-comment-thread/gr-comment-thread.ts b/polygerrit-ui/app/elements/shared/gr-comment-thread/gr-comment-thread.ts
index a9b910d..a5b7df7 100644
--- a/polygerrit-ui/app/elements/shared/gr-comment-thread/gr-comment-thread.ts
+++ b/polygerrit-ui/app/elements/shared/gr-comment-thread/gr-comment-thread.ts
@@ -60,7 +60,7 @@
 import {KnownExperimentId} from '../../../services/flags/flags';
 import {DiffInfo, DiffPreferencesInfo} from '../../../types/diff';
 import {RenderPreferences} from '../../../api/diff';
-import {check, checkProperty} from '../../../utils/common-util';
+import {check, assertIsDefined} from '../../../utils/common-util';
 
 const UNRESOLVED_EXPAND_COUNT = 5;
 const NEWLINE_PATTERN = /\n/g;
@@ -334,8 +334,8 @@
   }
 
   _getUrlForViewDiff(comments: UIComment[]) {
-    checkProperty(!!this.changeNum, 'changeNum');
-    checkProperty(!!this.projectName, 'projectName');
+    assertIsDefined(this.changeNum, 'changeNum');
+    assertIsDefined(this.projectName, 'projectName');
     check(comments.length > 0, 'comment not found');
     return GerritNav.getUrlForComment(
       this.changeNum,
@@ -633,8 +633,8 @@
   }
 
   _handleCommentDiscard(e: Event) {
-    if (!this.changeNum) throw new Error('changeNum is missing');
-    if (!this.patchNum) throw new Error('patchNum is missing');
+    assertIsDefined(this.changeNum, 'changeNum');
+    assertIsDefined(this.patchNum, 'patchNum');
     const diffCommentEl = (dom(e) as EventApi).rootTarget as GrComment;
     const comment = diffCommentEl.comment;
     const idx = this._indexOf(comment, this.comments);
diff --git a/polygerrit-ui/app/elements/shared/gr-comment/gr-comment.ts b/polygerrit-ui/app/elements/shared/gr-comment/gr-comment.ts
index 898aff3..bf376f6 100644
--- a/polygerrit-ui/app/elements/shared/gr-comment/gr-comment.ts
+++ b/polygerrit-ui/app/elements/shared/gr-comment/gr-comment.ts
@@ -61,6 +61,7 @@
 import {OpenFixPreviewEventDetail} from '../../../types/events';
 import {fireAlert} from '../../../utils/event-util';
 import {pluralize} from '../../../utils/string-util';
+import {assertIsDefined} from '../../../utils/common-util';
 
 const STORAGE_DEBOUNCE_INTERVAL = 400;
 const TOAST_DEBOUNCE_INTERVAL = 200;
@@ -322,7 +323,7 @@
   }
 
   _handlePortedMessageClick() {
-    if (!this.comment) throw new Error('comment not set');
+    assertIsDefined(this.comment, 'comment');
     this.reporting.reportInteraction('navigate-to-original-comment', {
       line: this.comment.line,
       range: this.comment.range,
@@ -496,10 +497,8 @@
     // prior to it being saved.
     this.cancelDebouncer(DEBOUNCER_STORE);
 
-    if (!this.comment?.path) throw new Error('Cannot erase Draft Comment');
-    if (this.changeNum === undefined) {
-      throw new Error('undefined changeNum');
-    }
+    assertIsDefined(this.comment?.path, 'comment.path');
+    assertIsDefined(this.changeNum, 'changeNum');
     this.storage.eraseDraftComment({
       changeNum: this.changeNum,
       patchNum: this._getPatchNum(),
diff --git a/polygerrit-ui/app/elements/shared/gr-hovercard-account/gr-hovercard-account.ts b/polygerrit-ui/app/elements/shared/gr-hovercard-account/gr-hovercard-account.ts
index 592f7903..ddae8ea 100644
--- a/polygerrit-ui/app/elements/shared/gr-hovercard-account/gr-hovercard-account.ts
+++ b/polygerrit-ui/app/elements/shared/gr-hovercard-account/gr-hovercard-account.ts
@@ -45,6 +45,7 @@
 import {ReviewerState} from '../../../constants/constants';
 import {CURRENT} from '../../../utils/patch-set-util';
 import {isInvolved, isRemovableReviewer} from '../../../utils/change-util';
+import {assertIsDefined} from '../../../utils/common-util';
 
 @customElement('gr-hovercard-account')
 export class GrHovercardAccount extends GestureEventListeners(
@@ -163,7 +164,7 @@
   }
 
   _handleChangeReviewerOrCCStatus() {
-    if (!this.change) throw new Error('expected change object to be present');
+    assertIsDefined(this.change, 'change');
     // accountKey() throws an error if _account_id & email is not found, which
     // we want to check before showing reloading toast
     const _accountKey = accountKey(this.account);
diff --git a/polygerrit-ui/app/elements/shared/gr-js-api-interface/gr-annotation-actions-context.ts b/polygerrit-ui/app/elements/shared/gr-js-api-interface/gr-annotation-actions-context.ts
index 6a4da7b..9f9ba6a 100644
--- a/polygerrit-ui/app/elements/shared/gr-js-api-interface/gr-annotation-actions-context.ts
+++ b/polygerrit-ui/app/elements/shared/gr-js-api-interface/gr-annotation-actions-context.ts
@@ -29,14 +29,13 @@
  * @param lineNumberEl The TD element of the line number to
  * apply the annotation to using annotateLineNumber.
  * @param line The line object.
- * @param path The file path (eg: /COMMIT_MSG').
+ * @param path The file path (eg: '/COMMIT_MSG').
  * @param changeNum The Gerrit change number.
- * @param patchNum The Gerrit patch number.
  */
 export class GrAnnotationActionsContext implements AnnotationContext {
-  private _contentEl: HTMLElement;
+  contentEl: HTMLElement;
 
-  private _lineNumberEl: HTMLElement;
+  lineNumberEl: HTMLElement;
 
   line: GrDiffLine;
 
@@ -53,9 +52,8 @@
     path: string,
     changeNum: string | number
   ) {
-    this._contentEl = contentEl;
-    this._lineNumberEl = lineNumberEl;
-
+    this.contentEl = contentEl;
+    this.lineNumberEl = lineNumberEl;
     this.line = line;
     this.path = path;
     this.changeNum = Number(changeNum);
@@ -80,12 +78,12 @@
     styleObject: GrStyleObject,
     side: string
   ) {
-    if (this._contentEl?.getAttribute('data-side') === side) {
+    if (this.contentEl?.getAttribute('data-side') === side) {
       GrAnnotation.annotateElement(
-        this._contentEl,
+        this.contentEl,
         offset,
         length,
-        styleObject.getClassName(this._contentEl)
+        styleObject.getClassName(this.contentEl)
       );
     }
   }
@@ -97,8 +95,8 @@
    * @param side The side of the update. ('left' or 'right')
    */
   annotateLineNumber(styleObject: GrStyleObject, side: string) {
-    if (this._lineNumberEl?.classList.contains(side)) {
-      styleObject.apply(this._lineNumberEl);
+    if (this.lineNumberEl?.classList.contains(side)) {
+      styleObject.apply(this.lineNumberEl);
     }
   }
 }
diff --git a/polygerrit-ui/app/elements/shared/gr-js-api-interface/gr-annotation-actions-js-api.ts b/polygerrit-ui/app/elements/shared/gr-js-api-interface/gr-annotation-actions-js-api.ts
index 4abc6e1..95252cf 100644
--- a/polygerrit-ui/app/elements/shared/gr-js-api-interface/gr-annotation-actions-js-api.ts
+++ b/polygerrit-ui/app/elements/shared/gr-js-api-interface/gr-annotation-actions-js-api.ts
@@ -21,65 +21,37 @@
 import {EventType, PluginApi} from '../../../api/plugin';
 import {appContext} from '../../../services/app-context';
 import {
-  AddLayerFunc,
+  AnnotationCallback,
   AnnotationPluginApi,
   CoverageProvider,
-  NotifyFunc,
 } from '../../../api/annotation';
 
 export class GrAnnotationActionsInterface implements AnnotationPluginApi {
-  // Collect all annotation layers instantiated by getLayer. Will be used when
-  // notifying their listeners in the notify function.
+  /**
+   * Collect all annotation layers instantiated by createLayer. This is only
+   * used for being able to look up the appropriate layer when notify() is
+   * being called by plugins.
+   */
   private annotationLayers: AnnotationLayer[] = [];
 
-  private coverageProvider: CoverageProvider | null = null;
+  private coverageProvider?: CoverageProvider;
 
-  // Default impl is a no-op.
-  private addLayerFunc: AddLayerFunc = () => {};
+  private annotationCallback?: AnnotationCallback;
 
-  reporting = appContext.reportingService;
+  private readonly reporting = appContext.reportingService;
 
   constructor(private readonly plugin: PluginApi) {
-    // Return this instance when there is an annotatediff event.
     plugin.on(EventType.ANNOTATE_DIFF, this);
   }
 
-  /**
-   * Register a function to call to apply annotations. Plugins should use
-   * GrAnnotationActionsContext.annotateRange and
-   * GrAnnotationActionsContext.annotateLineNumber to apply a CSS class to the
-   * line content or the line number.
-   *
-   * @param addLayerFunc The function
-   * that will be called when the AnnotationLayer is ready to annotate.
-   */
-  addLayer(addLayerFunc: AddLayerFunc) {
-    this.addLayerFunc = addLayerFunc;
+  setLayer(annotationCallback: AnnotationCallback) {
+    if (this.annotationCallback) {
+      console.warn('Overwriting an existing plugin annotation layer.');
+    }
+    this.annotationCallback = annotationCallback;
     return this;
   }
 
-  /**
-   * The specified function will be called with a notify function for the plugin
-   * to call when it has all required data for annotation. Optional.
-   *
-   * @param notifyFunc See doc of the notify function below to see what it does.
-   */
-  addNotifier(notifyFunc: (n: NotifyFunc) => void) {
-    notifyFunc(
-      (path: string, startRange: number, endRange: number, side: Side) =>
-        this.notify(path, startRange, endRange, side)
-    );
-    return this;
-  }
-
-  /**
-   * The specified function will be called when a gr-diff component is built,
-   * and feeds the returned coverage data into the diff. Optional.
-   *
-   * Be sure to call this only once and only from one plugin. Multiple coverage
-   * providers are not supported. A second call will just overwrite the
-   * provider of the first call.
-   */
   setCoverageProvider(
     coverageProvider: CoverageProvider
   ): GrAnnotationActionsInterface {
@@ -98,23 +70,6 @@
     return this.coverageProvider;
   }
 
-  /**
-   * Returns a checkbox HTMLElement that can be used to toggle annotations
-   * on/off. The checkbox will be initially disabled. Plugins should enable it
-   * when data is ready and should add a click handler to toggle CSS on/off.
-   *
-   * Note1: Calling this method from multiple plugins will only work for the
-   * 1st call. It will print an error message for all subsequent calls
-   * and will not invoke their onAttached functions.
-   * Note2: This method will be deprecated and eventually removed when
-   * https://bugs.chromium.org/p/gerrit/issues/detail?id=8077 is
-   * implemented.
-   *
-   * @param checkboxLabel Will be used as the label for the checkbox.
-   * Optional. "Enable" is used if this is not specified.
-   * @param onAttached The function that will be called
-   * when the checkbox is attached to the page.
-   */
   enableToggleCheckbox(
     checkboxLabel: string,
     onAttached: (checkboxEl: Element | null) => void
@@ -148,16 +103,6 @@
     return this;
   }
 
-  /**
-   * The notify function will call the listeners of all required annotation
-   * layers. Intended to be called by the plugin when all required data for
-   * annotation is available.
-   *
-   * @param path The file path whose listeners should be notified.
-   * @param start The line where the update starts.
-   * @param end The line where the update ends.
-   * @param side The side of the update ('left' or 'right').
-   */
   notify(path: string, start: number, end: number, side: Side) {
     for (const annotationLayer of this.annotationLayers) {
       // Notify only the annotation layer that is associated with the specified
@@ -169,24 +114,25 @@
   }
 
   /**
-   * Should be called to register annotation layers by the framework. Not
-   * intended to be called by plugins.
+   * Factory method called by Gerrit for creating a DiffLayer for each diff that
+   * is rendered.
    *
-   * Don't forget to dispose layer.
-   *
-   * @param path The file path (eg: /COMMIT_MSG').
-   * @param changeNum The Gerrit change number.
+   * Don't forget to also call disposeLayer().
    */
-  getLayer(path: string, changeNum: number) {
+  createLayer(path: string, changeNum: number) {
+    if (!this.annotationCallback) return undefined;
     const annotationLayer = new AnnotationLayer(
       path,
       changeNum,
-      this.addLayerFunc
+      this.annotationCallback
     );
     this.annotationLayers.push(annotationLayer);
     return annotationLayer;
   }
 
+  /**
+   * Called by Gerrit for each diff renderer that had called createLayer().
+   */
   disposeLayer(path: string) {
     this.annotationLayers = this.annotationLayers.filter(
       annotationLayer => annotationLayer.path !== path
@@ -194,6 +140,10 @@
   }
 }
 
+/**
+ * An AnnotationLayer exists for each file that is being rendered. This class is
+ * not exposed to plugins, but being used by Gerrit's diff rendering.
+ */
 export class AnnotationLayer implements DiffLayer {
   private listeners: DiffLayerListener[] = [];
 
@@ -202,13 +152,13 @@
    *
    * @param path The file path (eg: /COMMIT_MSG').
    * @param changeNum The Gerrit change number.
-   * @param addLayerFunc The function
+   * @param annotationCallback The function
    * that will be called when the AnnotationLayer is ready to annotate.
    */
   constructor(
     readonly path: string,
     private readonly changeNum: number,
-    private readonly addLayerFunc: AddLayerFunc
+    private readonly annotationCallback: AnnotationCallback
   ) {
     this.listeners = [];
   }
@@ -230,7 +180,8 @@
   }
 
   /**
-   * Layer method to add annotations to a line.
+   * Called by Gerrit during diff rendering for each line. Delegates to the
+   * plugin provided callback for potentially annotating this line.
    *
    * @param contentEl The DIV.contentText element of the line
    * content to apply the annotation to using annotateRange.
@@ -243,18 +194,20 @@
     lineNumberEl: HTMLElement,
     line: GrDiffLine
   ) {
-    const annotationActionsContext = new GrAnnotationActionsContext(
+    const context = new GrAnnotationActionsContext(
       contentEl,
       lineNumberEl,
       line,
       this.path,
       this.changeNum
     );
-    this.addLayerFunc(annotationActionsContext);
+    this.annotationCallback(context);
   }
 
   /**
-   * Notify Layer listeners of changes to annotations.
+   * Notify layer listeners (which typically is just Gerrit's diff renderer) of
+   * changes to annotations after the diff rendering had already completed. This
+   * is indirectly called by plugins using the AnnotationPluginApi.notify().
    *
    * @param start The line where the update starts.
    * @param end The line where the update ends.
diff --git a/polygerrit-ui/app/elements/shared/gr-js-api-interface/gr-annotation-actions-js-api_test.js b/polygerrit-ui/app/elements/shared/gr-js-api-interface/gr-annotation-actions-js-api_test.js
index 7ae34cf..9811f99 100644
--- a/polygerrit-ui/app/elements/shared/gr-js-api-interface/gr-annotation-actions-js-api_test.js
+++ b/polygerrit-ui/app/elements/shared/gr-js-api-interface/gr-annotation-actions-js-api_test.js
@@ -59,9 +59,9 @@
       assert.equal(context.line, line);
       assert.equal(context.changeNum, changeNum);
     };
-    annotationActions.addLayer(testLayerFunc);
+    annotationActions.setLayer(testLayerFunc);
 
-    const annotationLayer = annotationActions.getLayer(
+    const annotationLayer = annotationActions.createLayer(
         '/dummy/path', changeNum);
 
     const lineNumberEl = document.createElement('td');
@@ -72,27 +72,19 @@
   test('add notifier', () => {
     const path1 = '/dummy/path1';
     const path2 = '/dummy/path2';
-    const annotationLayer1 = annotationActions.getLayer(path1, 1);
-    const annotationLayer2 = annotationActions.getLayer(path2, 1);
+    annotationActions.setLayer(context => {});
+    const annotationLayer1 = annotationActions.createLayer(path1, 1);
+    const annotationLayer2 = annotationActions.createLayer(path2, 1);
     const layer1Spy = sinon.spy(annotationLayer1, 'notifyListeners');
     const layer2Spy = sinon.spy(annotationLayer2, 'notifyListeners');
 
-    let notify;
-    let notifyFuncCalled;
-    const notifyFunc = n => {
-      notifyFuncCalled = true;
-      notify = n;
-    };
-    annotationActions.addNotifier(notifyFunc);
-    assert.isTrue(notifyFuncCalled);
-
     // Assert that no layers are invoked with a different path.
-    notify('/dummy/path3', 0, 10, 'right');
+    annotationActions.notify('/dummy/path3', 0, 10, 'right');
     assert.isFalse(layer1Spy.called);
     assert.isFalse(layer2Spy.called);
 
     // Assert that only the 1st layer is invoked with path1.
-    notify(path1, 0, 10, 'right');
+    annotationActions.notify(path1, 0, 10, 'right');
     assert.isTrue(layer1Spy.called);
     assert.isFalse(layer2Spy.called);
 
@@ -101,7 +93,7 @@
     layer2Spy.resetHistory();
 
     // Assert that only the 2nd layer is invoked with path2.
-    notify(path2, 0, 20, 'left');
+    annotationActions.notify(path2, 0, 20, 'left');
     assert.isFalse(layer1Spy.called);
     assert.isTrue(layer2Spy.called);
   });
@@ -143,7 +135,8 @@
   });
 
   test('layer notify listeners', () => {
-    const annotationLayer = annotationActions.getLayer('/dummy/path', 1);
+    annotationActions.setLayer(context => {});
+    const annotationLayer = annotationActions.createLayer('/dummy/path', 1);
     let listenerCalledTimes = 0;
     const startRange = 10;
     const endRange = 20;
diff --git a/polygerrit-ui/app/elements/shared/gr-js-api-interface/gr-js-api-interface-element.ts b/polygerrit-ui/app/elements/shared/gr-js-api-interface/gr-js-api-interface-element.ts
index 830fb92..8689ad2 100644
--- a/polygerrit-ui/app/elements/shared/gr-js-api-interface/gr-js-api-interface-element.ts
+++ b/polygerrit-ui/app/elements/shared/gr-js-api-interface/gr-js-api-interface-element.ts
@@ -252,8 +252,8 @@
     for (const cb of this._getEventCallbacks(EventType.ANNOTATE_DIFF)) {
       const annotationApi = (cb as unknown) as GrAnnotationActionsInterface;
       try {
-        const layer = annotationApi.getLayer(path, changeNum);
-        layers.push(layer);
+        const layer = annotationApi.createLayer(path, changeNum);
+        if (layer) layers.push(layer);
       } catch (err) {
         this.reporting.error(err);
       }
diff --git a/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-rest-api-interface.ts b/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-rest-api-interface.ts
index f3be790..ec59ddc 100644
--- a/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-rest-api-interface.ts
+++ b/polygerrit-ui/app/elements/shared/gr-rest-api-interface/gr-rest-api-interface.ts
@@ -2234,13 +2234,13 @@
     if (!basePatchNum && !patchNum && !path) {
       return this._getDiffComments(changeNum, '/comments', {
         'enable-context': true,
-        'context-padding': 5,
+        'context-padding': 3,
       });
     }
     return this._getDiffComments(
       changeNum,
       '/comments',
-      {'enable-context': true, 'context-padding': 5},
+      {'enable-context': true, 'context-padding': 3},
       basePatchNum,
       patchNum,
       path
diff --git a/polygerrit-ui/app/samples/coverage-plugin.js b/polygerrit-ui/app/samples/coverage-plugin.js
index 9b2b687..8d321c7 100644
--- a/polygerrit-ui/app/samples/coverage-plugin.js
+++ b/polygerrit-ui/app/samples/coverage-plugin.js
@@ -41,7 +41,7 @@
   const coverageStyle = styleApi.css('background-color: #EF9B9B !important');
   const emptyStyle = styleApi.css('');
 
-  annotationApi.addLayer(context => {
+  annotationApi.setLayer(context => {
     if (Object.keys(coverageData).length === 0) {
       // Coverage data is not ready yet.
       return;
@@ -64,19 +64,13 @@
       }
     }
   }).enableToggleCheckbox('Display Coverage', checkbox => {
-    // Checkbox is attached so now add the notifier that will be controlled
-    // by the checkbox.
-    // Checkbox will only be added to the file diff page, in the top right
-    // section near the "Diff view".
-    annotationApi.addNotifier(notifyFunc => {
-      populateWithDummyData(coverageData);
-      checkbox.disabled = false;
-      checkbox.onclick = e => {
-        displayCoverage = e.target.checked;
-        Object.keys(coverageData).forEach(file => {
-          notifyFunc(file, 0, coverageData[file].totalLines, 'right');
-        });
-      };
-    });
+    populateWithDummyData(coverageData);
+    checkbox.disabled = false;
+    checkbox.onclick = e => {
+      displayCoverage = e.target.checked;
+      Object.keys(coverageData).forEach(file => {
+        annotationApi.notify(file, 0, coverageData[file].totalLines, 'right');
+      });
+    };
   });
-});
\ No newline at end of file
+});
diff --git a/polygerrit-ui/app/test/common-test-setup.ts b/polygerrit-ui/app/test/common-test-setup.ts
index 0f394be..4e1662c 100644
--- a/polygerrit-ui/app/test/common-test-setup.ts
+++ b/polygerrit-ui/app/test/common-test-setup.ts
@@ -90,8 +90,10 @@
 }
 
 window.fixture = fixtureImpl;
+let testSetupTimestampMs = 0;
 
 setup(() => {
+  testSetupTimestampMs = new Date().getTime();
   window.Gerrit = {};
   initGlobalVariables();
   addIronOverlayBackdropStyleEl();
@@ -201,4 +203,9 @@
   // `this.debounce()`. For those please be careful and cancel them using
   // `this.cancelDebouncer()` in the `detached()` lifecycle hook.
   flushDebouncers();
+  const testTeardownTimestampMs = new Date().getTime();
+  const elapsedMs = testTeardownTimestampMs - testSetupTimestampMs;
+  if (elapsedMs > 1000) {
+    console.warn(`ATTENTION! Test took longer than 1 second: ${elapsedMs} ms`);
+  }
 });
diff --git a/polygerrit-ui/app/utils/comment-util.ts b/polygerrit-ui/app/utils/comment-util.ts
index 61456a2..de12a2a 100644
--- a/polygerrit-ui/app/utils/comment-util.ts
+++ b/polygerrit-ui/app/utils/comment-util.ts
@@ -289,6 +289,8 @@
 }
 
 export function computeDiffFromContext(context: ContextLine[], path: string) {
+  // do not render more than 20 lines of context
+  context = context.slice(0, 20);
   const diff: DiffInfo = {
     meta_a: {
       name: '',
diff --git a/polygerrit-ui/app/utils/common-util.ts b/polygerrit-ui/app/utils/common-util.ts
index ad76b79..f95105d 100644
--- a/polygerrit-ui/app/utils/common-util.ts
+++ b/polygerrit-ui/app/utils/common-util.ts
@@ -67,6 +67,18 @@
 }
 
 /**
+ * Throws an error if the property is not defined.
+ */
+export function assertIsDefined<T>(
+  val: T,
+  variableName = 'variable'
+): asserts val is NonNullable<T> {
+  if (val === undefined || val === null) {
+    throw new Error(`${variableName} is not defined`);
+  }
+}
+
+/**
  * Returns true, if both sets contain the same members.
  */
 export function areSetsEqual<T>(a: Set<T>, b: Set<T>): boolean {