diff --git a/polygerrit-ui/app/elements/core/gr-router/gr-router.ts b/polygerrit-ui/app/elements/core/gr-router/gr-router.ts
index 28b7e3b..ac54d35 100644
--- a/polygerrit-ui/app/elements/core/gr-router/gr-router.ts
+++ b/polygerrit-ui/app/elements/core/gr-router/gr-router.ts
@@ -10,7 +10,11 @@
 } from '../../../utils/page-wrapper-utils';
 import {NavigationService} from '../gr-navigation/gr-navigation';
 import {getAppContext} from '../../../services/app-context';
-import {convertToPatchSetNum} from '../../../utils/patch-set-util';
+import {
+  computeAllPatchSets,
+  computeLatestPatchNum,
+  convertToPatchSetNum,
+} from '../../../utils/patch-set-util';
 import {assertIsDefined} from '../../../utils/common-util';
 import {
   BasePatchSetNum,
@@ -27,7 +31,7 @@
 import {AppElement, AppElementParams} from '../../gr-app-types';
 import {LocationChangeEventDetail} from '../../../types/events';
 import {GerritView, RouterModel} from '../../../services/router/router-model';
-import {firePageError} from '../../../utils/event-util';
+import {fireAlert, firePageError} from '../../../utils/event-util';
 import {windowLocationReload} from '../../../utils/dom-util';
 import {
   getBaseUrl,
@@ -87,6 +91,14 @@
 import {SearchViewModel, SearchViewState} from '../../../models/views/search';
 import {DashboardSection} from '../../../utils/dashboard-util';
 import {Subscription} from 'rxjs';
+import {
+  addPath,
+  findComment,
+  getPatchRangeForCommentUrl,
+  isInBaseOfPatchRange,
+} from '../../../utils/comment-util';
+import {createDiffUrl} from '../../../models/views/diff';
+import {isFileUnchanged} from '../../../embed/diff/gr-diff/gr-diff-utils';
 
 const RoutePattern = {
   ROOT: '/',
@@ -1470,22 +1482,57 @@
     this.changeViewModel.setState(state);
   }
 
-  handleCommentRoute(ctx: PageContext) {
+  async handleCommentRoute(ctx: PageContext) {
     const changeNum = Number(ctx.params[1]) as NumericChangeId;
-    const state: ChangeViewState = {
-      repo: ctx.params[0] as RepoName,
+    const repo = ctx.params[0] as RepoName;
+    const commentId = ctx.params[2] as UrlEncodedCommentId;
+
+    const comments = await this.restApiService.getDiffComments(changeNum);
+    const change = await this.restApiService.getChangeDetail(changeNum);
+
+    const comment = findComment(addPath(comments), commentId);
+    const path = comment?.path;
+    const patchsets = computeAllPatchSets(change);
+    const latestPatchNum = computeLatestPatchNum(patchsets);
+    if (!comment || !path || !latestPatchNum) {
+      this.show404();
+      return;
+    }
+    let {basePatchNum, patchNum} = getPatchRangeForCommentUrl(
+      comment,
+      latestPatchNum
+    );
+
+    if (basePatchNum !== PARENT) {
+      const diff = await this.restApiService.getDiff(
+        changeNum,
+        basePatchNum,
+        patchNum,
+        path
+      );
+      if (diff && isFileUnchanged(diff)) {
+        fireAlert(
+          document,
+          `File is unchanged between Patchset ${basePatchNum} and ${patchNum}.
+           Showing diff of Base vs ${basePatchNum}.`
+        );
+        patchNum = basePatchNum as RevisionPatchSetNum;
+        basePatchNum = PARENT;
+      }
+    }
+
+    const diffUrl = createDiffUrl({
       changeNum,
-      commentId: ctx.params[2] as UrlEncodedCommentId,
-      view: GerritView.CHANGE,
-      childView: ChangeChildView.DIFF,
-      diffView: {commentLink: true},
-    };
-    this.reporting.setRepoName(state.repo ?? '');
-    this.reporting.setChangeId(changeNum);
-    this.normalizePatchRangeParams(state);
-    // Note that router model view must be updated before view models.
-    this.setState(state);
-    this.changeViewModel.setState(state);
+      repo,
+      patchNum,
+      basePatchNum,
+      diffView: {
+        path,
+        lineNum: comment.line,
+        leftSide: isInBaseOfPatchRange(comment, {basePatchNum, patchNum}),
+      },
+    });
+    this.redirect(diffUrl);
   }
 
   handleCommentsRoute(ctx: PageContext) {
diff --git a/polygerrit-ui/app/elements/core/gr-router/gr-router_test.ts b/polygerrit-ui/app/elements/core/gr-router/gr-router_test.ts
index ce6e933..d8761bf 100644
--- a/polygerrit-ui/app/elements/core/gr-router/gr-router_test.ts
+++ b/polygerrit-ui/app/elements/core/gr-router/gr-router_test.ts
@@ -31,6 +31,13 @@
 import {ChangeChildView, ChangeViewState} from '../../../models/views/change';
 import {PatchRangeParams} from '../../../utils/url-util';
 import {testResolver} from '../../../test/common-test-setup';
+import {
+  createComment,
+  createDiff,
+  createParsedChange,
+  createRevision,
+} from '../../../test/test-data-generators';
+import {ParsedChangeInfo} from '../../../types/types';
 
 suite('gr-router tests', () => {
   let router: GrRouter;
@@ -1209,25 +1216,74 @@
           assert.isFalse(redirectStub.called);
         });
 
-        test('comment route', () => {
-          const url = '/c/gerrit/+/264833/comment/00049681_f34fd6a9/';
+        test('comment route base..1', async () => {
+          const change: ParsedChangeInfo = createParsedChange();
+          const repo = change.project;
+          const changeNum = change._number;
+          const ps = 1 as RevisionPatchSetNum;
+          const line = 23;
+          const id = '00049681_f34fd6a9' as UrlEncodedCommentId;
+          stubRestApi('getChangeDetail').resolves(change);
+          stubRestApi('getDiffComments').resolves({
+            filepath: [{...createComment(), id, patch_set: ps, line}],
+          });
+
+          const url = `/c/${repo}/+/${changeNum}/comment/${id}/`;
           const groups = url.match(_testOnly_RoutePattern.COMMENT);
-          assert.deepEqual(groups!.slice(1), [
-            'gerrit', // project
-            '264833', // changeNum
-            '00049681_f34fd6a9', // commentId
-          ]);
-          assertctxToParams(
-            {params: groups!.slice(1)} as any,
-            'handleCommentRoute',
-            {
-              repo: 'gerrit' as RepoName,
-              changeNum: 264833 as NumericChangeId,
-              commentId: '00049681_f34fd6a9' as UrlEncodedCommentId,
-              view: GerritView.CHANGE,
-              childView: ChangeChildView.DIFF,
-              diffView: {commentLink: true},
-            }
+          assert.deepEqual(groups!.slice(1), [repo, `${changeNum}`, id]);
+
+          await router.handleCommentRoute({params: groups!.slice(1)} as any);
+          assert.isTrue(redirectStub.calledOnce);
+          assert.equal(
+            redirectStub.lastCall.args[0],
+            `/c/${repo}/+/${changeNum}/${ps}/filepath#${line}`
+          );
+        });
+
+        test('comment route 1..2', async () => {
+          const change: ParsedChangeInfo = {
+            ...createParsedChange(),
+            revisions: {
+              abc: createRevision(1),
+              def: createRevision(2),
+            },
+          };
+          const repo = change.project;
+          const changeNum = change._number;
+          const ps = 1 as RevisionPatchSetNum;
+          const line = 23;
+          const id = '00049681_f34fd6a9' as UrlEncodedCommentId;
+
+          stubRestApi('getChangeDetail').resolves(change);
+          stubRestApi('getDiffComments').resolves({
+            filepath: [{...createComment(), id, patch_set: ps, line}],
+          });
+          const diffStub = stubRestApi('getDiff');
+
+          const url = `/c/${repo}/+/${changeNum}/comment/${id}/`;
+          const groups = url.match(_testOnly_RoutePattern.COMMENT);
+
+          // If getDiff() returns a diff with changes, then we will compare
+          // the patchset of the comment (1) against latest (2).
+          diffStub.onFirstCall().resolves(createDiff());
+          await router.handleCommentRoute({params: groups!.slice(1)} as any);
+          assert.isTrue(redirectStub.calledOnce);
+          assert.equal(
+            redirectStub.lastCall.args[0],
+            `/c/${repo}/+/${changeNum}/${ps}..2/filepath#b${line}`
+          );
+
+          // If getDiff() returns an unchanged diff, then we will compare
+          // the patchset of the comment (1) against base.
+          diffStub.onSecondCall().resolves({
+            ...createDiff(),
+            content: [],
+          });
+          await router.handleCommentRoute({params: groups!.slice(1)} as any);
+          assert.isTrue(redirectStub.calledTwice);
+          assert.equal(
+            redirectStub.lastCall.args[0],
+            `/c/${repo}/+/${changeNum}/${ps}/filepath#${line}`
           );
         });
 
diff --git a/polygerrit-ui/app/elements/diff/gr-comment-api/gr-comment-api.ts b/polygerrit-ui/app/elements/diff/gr-comment-api/gr-comment-api.ts
index 26043b32..6eb5243 100644
--- a/polygerrit-ui/app/elements/diff/gr-comment-api/gr-comment-api.ts
+++ b/polygerrit-ui/app/elements/diff/gr-comment-api/gr-comment-api.ts
@@ -7,7 +7,6 @@
   PatchRange,
   PatchSetNum,
   RobotCommentInfo,
-  UrlEncodedCommentId,
   PathToCommentsInfoMap,
   FileInfo,
   PARENT,
@@ -64,26 +63,6 @@
     return this._drafts;
   }
 
-  findCommentById(
-    commentId?: UrlEncodedCommentId
-  ): CommentInfo | DraftInfo | undefined {
-    if (!commentId) return undefined;
-    const findComment = (comments: {
-      [path: string]: (CommentInfo | DraftInfo)[];
-    }) => {
-      let comment;
-      for (const path of Object.keys(comments)) {
-        comment = comment || comments[path].find(c => c.id === commentId);
-      }
-      return comment;
-    };
-    return (
-      findComment(this._comments) ||
-      findComment(this._robotComments) ||
-      findComment(this._drafts)
-    );
-  }
-
   /**
    * Get an object mapping file paths to a boolean representing whether that
    * path contains diff comments in the given patch set (including drafts and
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 7f38c79..6fb19a0 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
@@ -52,7 +52,6 @@
   NumericChangeId,
   PARENT,
   PatchRange,
-  PatchSetNum,
   PatchSetNumber,
   PreferencesInfo,
   RepoName,
@@ -72,11 +71,7 @@
 import {CommentSide, DiffViewMode, Side} from '../../../constants/constants';
 import {GrApplyFixDialog} from '../gr-apply-fix-dialog/gr-apply-fix-dialog';
 import {RevisionInfo as RevisionInfoObj} from '../../shared/revision-info/revision-info';
-import {
-  CommentMap,
-  getPatchRangeForCommentUrl,
-  isInBaseOfPatchRange,
-} from '../../../utils/comment-util';
+import {CommentMap} from '../../../utils/comment-util';
 import {
   EventType,
   OpenFixPreviewEvent,
@@ -1455,44 +1450,6 @@
     this.initCursor(leftSide);
   }
 
-  // Private but used in tests.
-  displayDiffBaseAgainstLeftToast() {
-    if (!this.patchRange) return;
-    fireAlert(
-      this,
-      `Patchset ${this.patchRange.basePatchNum} vs ` +
-        `${this.patchRange.patchNum} selected. Press v + \u2190 to view ` +
-        `Base vs ${this.patchRange.basePatchNum}`
-    );
-  }
-
-  private displayDiffAgainstLatestToast(latestPatchNum?: PatchSetNum) {
-    if (!this.patchRange) return;
-    const leftPatchset =
-      this.patchRange.basePatchNum === PARENT
-        ? 'Base'
-        : `Patchset ${this.patchRange.basePatchNum}`;
-    fireAlert(
-      this,
-      `${leftPatchset} vs
-            ${this.patchRange.patchNum} selected\n. Press v + \u2191 to view
-            ${leftPatchset} vs Patchset ${latestPatchNum}`
-    );
-  }
-
-  private displayToasts() {
-    if (!this.patchRange) return;
-    if (this.patchRange.basePatchNum !== PARENT) {
-      this.displayDiffBaseAgainstLeftToast();
-      return;
-    }
-    const latestPatchNum = computeLatestPatchNum(this.allPatchSets);
-    if (this.patchRange.patchNum !== latestPatchNum) {
-      this.displayDiffAgainstLatestToast(latestPatchNum);
-      return;
-    }
-  }
-
   private initCommitRange() {
     let commit: CommitId | undefined;
     let baseCommit: CommitId | undefined;
@@ -1539,58 +1496,25 @@
     let leftSide = false;
     if (!this.change) return;
     if (this.viewState?.childView !== ChangeChildView.DIFF) return;
-    if (this.viewState?.commentId) {
-      const comment = this.changeComments?.findCommentById(
-        this.viewState.commentId
-      );
-      if (!comment) {
-        fireAlert(this, 'comment not found');
-        this.getNavigation().setUrl(createChangeUrl({change: this.change}));
-        return;
-      }
-      this.getChangeModel().updatePath(comment.path);
-
-      const latestPatchNum = computeLatestPatchNum(this.allPatchSets);
-      if (!latestPatchNum) throw new Error('Missing allPatchSets');
-      this.patchRange = getPatchRangeForCommentUrl(comment, latestPatchNum);
-      leftSide = isInBaseOfPatchRange(comment, this.patchRange);
-
-      this.focusLineNum = comment.line;
-    } else {
-      if (this.viewState.diffView?.path) {
-        this.getChangeModel().updatePath(this.viewState.diffView.path);
-      }
-      if (this.viewState.patchNum) {
-        this.patchRange = {
-          patchNum: this.viewState.patchNum,
-          basePatchNum: this.viewState.basePatchNum || PARENT,
-        };
-      }
-      if (this.viewState.diffView?.lineNum) {
-        this.focusLineNum = this.viewState.diffView.lineNum;
-        leftSide = !!this.viewState.diffView?.leftSide;
-      }
+    if (this.viewState.diffView?.path) {
+      this.getChangeModel().updatePath(this.viewState.diffView.path);
+    }
+    if (this.viewState.patchNum) {
+      this.patchRange = {
+        patchNum: this.viewState.patchNum,
+        basePatchNum: this.viewState.basePatchNum || PARENT,
+      };
+    }
+    if (this.viewState.diffView?.lineNum) {
+      this.focusLineNum = this.viewState.diffView.lineNum;
+      leftSide = !!this.viewState.diffView?.leftSide;
     }
     assertIsDefined(this.patchRange, 'patchRange');
     this.initLineOfInterestAndCursor(leftSide);
 
-    if (this.viewState?.commentId) {
-      // url is of type /comment/{commentId} which isn't meaningful
-      this.updateUrlToDiffUrl(this.focusLineNum, leftSide);
-    }
-
     this.commentMap = this.getPaths();
   }
 
-  // Private but used in tests.
-  isFileUnchanged(diff?: DiffInfo) {
-    if (!diff || !diff.content) return false;
-    return !diff.content.some(
-      content =>
-        (content.a && !content.common) || (content.b && !content.common)
-    );
-  }
-
   private isSameDiffLoaded(value: ChangeViewState) {
     return (
       this.patchRange?.basePatchNum === value.basePatchNum &&
@@ -1660,8 +1584,7 @@
     // When navigating away from the page, there is a possibility that the
     // patch number is no longer a part of the URL (say when navigating to
     // the top-level change info view) and therefore undefined in `params`.
-    // If route is of type /comment/<commentId>/ then no patchNum is present
-    if (!viewState.patchNum && !viewState.diffView?.commentLink) {
+    if (!viewState.patchNum) {
       this.reporting.error(
         'GrDiffView',
         new Error(`Invalid diff view URL, no patchNum found: ${this.viewState}`)
@@ -1691,38 +1614,6 @@
         this.reporting.diffViewDisplayed();
       })
       .then(() => {
-        const fileUnchanged = this.isFileUnchanged(this.diff);
-        if (fileUnchanged && viewState.diffView?.commentLink) {
-          assertIsDefined(this.change, 'change');
-          assertIsDefined(this.path, 'path');
-          assertIsDefined(this.patchRange, 'patchRange');
-
-          if (this.patchRange.basePatchNum === PARENT) {
-            // file is unchanged between Base vs X
-            // hence should not show diff between Base vs Base
-            return;
-          }
-
-          fireAlert(
-            this,
-            `File is unchanged between Patchset
-                  ${this.patchRange.basePatchNum} and
-                  ${this.patchRange.patchNum}. Showing diff of Base vs
-                  ${this.patchRange.basePatchNum}`
-          );
-          this.getNavigation().setUrl(
-            createDiffUrl({
-              change: this.change,
-              patchNum: this.patchRange.basePatchNum as RevisionPatchSetNum,
-              basePatchNum: PARENT,
-              diffView: {path: this.path, lineNum: this.focusLineNum},
-            })
-          );
-          return;
-        }
-        if (viewState.diffView?.commentLink) {
-          this.displayToasts();
-        }
         // If the blame was loaded for a previous file and user navigates to
         // another file, then we load the blame for this file too
         if (this.isBlameLoaded) this.loadBlame();
@@ -2139,15 +2030,12 @@
       fireAlert(this, 'Left is already base.');
       return;
     }
-    const lineNum = this.viewState?.diffView?.commentLink
-      ? this.focusLineNum
-      : undefined;
     this.getNavigation().setUrl(
       createDiffUrl({
         change: this.change,
         patchNum: this.patchRange.basePatchNum as RevisionPatchSetNum,
         basePatchNum: PARENT,
-        diffView: {path: this.path, lineNum},
+        diffView: {path: this.path},
       })
     );
   }
diff --git a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view_test.ts b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view_test.ts
index fbe999b..8e242f9 100644
--- a/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view_test.ts
+++ b/polygerrit-ui/app/elements/diff/gr-diff-view/gr-diff-view_test.ts
@@ -54,10 +54,10 @@
   UrlEncodedCommentId,
 } from '../../../types/common';
 import {CursorMoveResult} from '../../../api/core';
-import {DiffInfo, Side} from '../../../api/diff';
+import {Side} from '../../../api/diff';
 import {Files, GrDiffView} from './gr-diff-view';
 import {DropdownItem} from '../../shared/gr-dropdown-list/gr-dropdown-list';
-import {SinonFakeTimers, SinonStub, SinonSpy} from 'sinon';
+import {SinonFakeTimers, SinonStub} from 'sinon';
 import {
   changeModelToken,
   ChangeModel,
@@ -198,66 +198,6 @@
       });
     });
 
-    suite('comment route', () => {
-      let initLineOfInterestAndCursorStub: SinonStub;
-      let replaceStateStub: SinonStub;
-      let viewStateChangedSpy: SinonSpy;
-      setup(() => {
-        initLineOfInterestAndCursorStub = sinon.stub(
-          element,
-          'initLineOfInterestAndCursor'
-        );
-        replaceStateStub = sinon.stub(history, 'replaceState');
-        sinon.stub(element, 'fetchFiles');
-        stubReporting('diffViewDisplayed');
-        assertIsDefined(element.diffHost);
-        sinon.stub(element.diffHost, 'reload').returns(Promise.resolve());
-        viewStateChangedSpy = sinon.spy(element, 'viewStateChanged');
-        changeModel.setState({
-          change: {
-            ...createParsedChange(),
-            revisions: createRevisions(11),
-          },
-          loadingStatus: LoadingStatus.LOADED,
-        });
-      });
-
-      test('comment url resolves to comment.patch_set vs latest', () => {
-        commentsModel.setState({
-          comments: {
-            '/COMMIT_MSG': [
-              createComment('c1', 10, 2, '/COMMIT_MSG'),
-              createComment('c3', 10, PARENT, '/COMMIT_MSG'),
-            ],
-          },
-          robotComments: {},
-          drafts: {},
-          portedComments: {},
-          portedDrafts: {},
-          discardedDrafts: [],
-        });
-        element.viewState = {
-          ...createDiffViewState(),
-          commentId: 'c1' as UrlEncodedCommentId,
-          patchNum: 1 as RevisionPatchSetNum,
-          diffView: {path: 'abcd', commentLink: true},
-        };
-        element.change = {
-          ...createParsedChange(),
-          revisions: createRevisions(11),
-        };
-        return viewStateChangedSpy.returnValues[0].then(() => {
-          assert.isTrue(
-            initLineOfInterestAndCursorStub.calledWithExactly(true)
-          );
-          assert.equal(element.focusLineNum, 10);
-          assert.equal(element.patchRange?.patchNum, 11 as RevisionPatchSetNum);
-          assert.equal(element.patchRange?.basePatchNum, 2 as BasePatchSetNum);
-          assert.isTrue(replaceStateStub.called);
-        });
-      });
-    });
-
     test('viewState change causes blame to load if it was set to true', () => {
       // Blame loads for subsequent files if it was loaded for one file
       element.isBlameLoaded = true;
@@ -282,166 +222,6 @@
       });
     });
 
-    test('unchanged diff X vs latest from comment links navigates to base vs X', async () => {
-      commentsModel.setState({
-        comments: {
-          '/COMMIT_MSG': [
-            createComment('c1', 10, 2, '/COMMIT_MSG'),
-            createComment('c3', 10, PARENT, '/COMMIT_MSG'),
-          ],
-        },
-        robotComments: {},
-        drafts: {},
-        portedComments: {},
-        portedDrafts: {},
-        discardedDrafts: [],
-      });
-      stubReporting('diffViewDisplayed');
-      sinon.stub(element, 'loadBlame');
-      assertIsDefined(element.diffHost);
-      sinon.stub(element.diffHost, 'reload').returns(Promise.resolve());
-      sinon.stub(element, 'isFileUnchanged').returns(true);
-      const viewStateChangedSpy = sinon.spy(element, 'viewStateChanged');
-      changeModel.setState({
-        change: {
-          ...createParsedChange(),
-          revisions: createRevisions(11),
-        },
-        loadingStatus: LoadingStatus.LOADED,
-      });
-      element.viewState = {
-        ...createDiffViewState(),
-        commentId: 'c1' as UrlEncodedCommentId,
-        diffView: {path: '/COMMIT_MSG', commentLink: true},
-      };
-      element.change = {
-        ...createParsedChange(),
-        revisions: createRevisions(11),
-      };
-      await viewStateChangedSpy.returnValues[0];
-      assert.isTrue(setUrlStub.calledOnce);
-      assert.equal(
-        setUrlStub.lastCall.firstArg,
-        '/c/test-project/+/42/2//COMMIT_MSG#10'
-      );
-    });
-
-    test('unchanged diff Base vs latest from comment does not navigate', async () => {
-      commentsModel.setState({
-        comments: {
-          '/COMMIT_MSG': [
-            createComment('c1', 10, 2, '/COMMIT_MSG'),
-            createComment('c3', 10, PARENT, '/COMMIT_MSG'),
-          ],
-        },
-        robotComments: {},
-        drafts: {},
-        portedComments: {},
-        portedDrafts: {},
-        discardedDrafts: [],
-      });
-      stubReporting('diffViewDisplayed');
-      sinon.stub(element, 'loadBlame');
-      assertIsDefined(element.diffHost);
-      sinon.stub(element.diffHost, 'reload').returns(Promise.resolve());
-      sinon.stub(element, 'isFileUnchanged').returns(true);
-      const viewStateChangedSpy = sinon.spy(element, 'viewStateChanged');
-      changeModel.setState({
-        change: {
-          ...createParsedChange(),
-          revisions: createRevisions(11),
-        },
-        loadingStatus: LoadingStatus.LOADED,
-      });
-      element.viewState = {
-        ...createDiffViewState(),
-        commentId: 'c3' as UrlEncodedCommentId,
-        diffView: {path: '/COMMIT_MSG', commentLink: true},
-      };
-      element.change = {
-        ...createParsedChange(),
-        revisions: createRevisions(11),
-      };
-      await viewStateChangedSpy.returnValues[0];
-      assert.isFalse(setUrlStub.calledOnce);
-    });
-
-    test('isFileUnchanged', () => {
-      let diff: DiffInfo = {
-        ...createDiff(),
-        content: [
-          {a: ['abcd'], ab: ['ef']},
-          {b: ['ancd'], a: ['xx']},
-        ],
-      };
-      assert.equal(element.isFileUnchanged(diff), false);
-      diff = {
-        ...createDiff(),
-        content: [{ab: ['abcd']}, {ab: ['ancd']}],
-      };
-      assert.equal(element.isFileUnchanged(diff), true);
-      diff = {
-        ...createDiff(),
-        content: [
-          {a: ['abcd'], ab: ['ef'], common: true},
-          {b: ['ancd'], ab: ['xx']},
-        ],
-      };
-      assert.equal(element.isFileUnchanged(diff), false);
-      diff = {
-        ...createDiff(),
-        content: [
-          {a: ['abcd'], ab: ['ef'], common: true},
-          {b: ['ancd'], ab: ['xx'], common: true},
-        ],
-      };
-      assert.equal(element.isFileUnchanged(diff), true);
-    });
-
-    test('diff toast to go to latest is shown and not base', async () => {
-      commentsModel.setState({
-        comments: {
-          '/COMMIT_MSG': [
-            createComment('c1', 10, 2, '/COMMIT_MSG'),
-            createComment('c3', 10, PARENT, '/COMMIT_MSG'),
-          ],
-        },
-        robotComments: {},
-        drafts: {},
-        portedComments: {},
-        portedDrafts: {},
-        discardedDrafts: [],
-      });
-
-      stubReporting('diffViewDisplayed');
-      sinon.stub(element, 'loadBlame');
-      assertIsDefined(element.diffHost);
-      sinon.stub(element.diffHost, 'reload').returns(Promise.resolve());
-      const viewStateChangedSpy = sinon.spy(element, 'viewStateChanged');
-      element.change = undefined;
-      changeModel.setState({
-        change: {
-          ...createParsedChange(),
-          revisions: createRevisions(11),
-        },
-        loadingStatus: LoadingStatus.LOADED,
-      });
-      element.patchRange = {
-        patchNum: 2 as RevisionPatchSetNum,
-        basePatchNum: 1 as BasePatchSetNum,
-      };
-      sinon.stub(element, 'isFileUnchanged').returns(false);
-      const toastStub = sinon.stub(element, 'displayDiffBaseAgainstLeftToast');
-      element.viewState = {
-        ...createDiffViewState(),
-        repo: 'p' as RepoName,
-        commentId: 'c1' as UrlEncodedCommentId,
-        diffView: {commentLink: true},
-      };
-      await viewStateChangedSpy.returnValues[0];
-      assert.isTrue(toastStub.called);
-    });
-
     test('toggle left diff with a hotkey', () => {
       assertIsDefined(element.diffHost);
       const toggleLeftDiffStub = sinon.stub(element.diffHost, 'toggleLeftDiff');
@@ -875,28 +655,6 @@
       assert.equal(setUrlStub.lastCall.firstArg, '/c/test-project/+/42/1/foo');
     });
 
-    test('handleDiffBaseAgainstLeft when initially navigating to a comment', () => {
-      element.change = {
-        ...createParsedChange(),
-        revisions: createRevisions(10),
-      };
-      element.patchRange = {
-        patchNum: 3 as RevisionPatchSetNum,
-        basePatchNum: 1 as BasePatchSetNum,
-      };
-      sinon.stub(element, 'viewStateChanged');
-      element.viewState = {
-        ...createDiffViewState(),
-        diffView: {commentLink: true},
-      };
-      element.focusLineNum = 10;
-      element.handleDiffBaseAgainstLeft();
-      assert.equal(
-        setUrlStub.lastCall.firstArg,
-        '/c/test-project/+/42/1/some/path.txt#10'
-      );
-    });
-
     test('handleDiffRightAgainstLatest', async () => {
       element.path = 'foo';
       element.change = {
diff --git a/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-utils.ts b/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-utils.ts
index f583c2e..4174d1d 100644
--- a/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-utils.ts
+++ b/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-utils.ts
@@ -48,6 +48,12 @@
   }, 0);
 }
 
+export function isFileUnchanged(diff: DiffInfo) {
+  return !diff.content.some(
+    content => (content.a && !content.common) || (content.b && !content.common)
+  );
+}
+
 export function getResponsiveMode(
   prefs: DiffPreferencesInfo,
   renderPrefs?: RenderPreferences
diff --git a/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-utils_test.ts b/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-utils_test.ts
index 98b4586..25dc768 100644
--- a/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-utils_test.ts
+++ b/polygerrit-ui/app/embed/diff/gr-diff/gr-diff-utils_test.ts
@@ -4,8 +4,15 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 import {assert} from '@open-wc/testing';
+import {DiffInfo} from '../../../api/diff';
 import '../../../test/common-test-setup';
-import {createElementDiff, formatText, createTabWrapper} from './gr-diff-utils';
+import {createDiff} from '../../../test/test-data-generators';
+import {
+  createElementDiff,
+  formatText,
+  createTabWrapper,
+  isFileUnchanged,
+} from './gr-diff-utils';
 
 const LINE_BREAK_HTML = '<span class="gr-diff br"></span>';
 
@@ -156,4 +163,36 @@
     expectTextLength('abc\tde\t', 10, 20);
     expectTextLength('\t\t\t\t\t', 20, 100);
   });
+
+  test('isFileUnchanged', () => {
+    let diff: DiffInfo = {
+      ...createDiff(),
+      content: [
+        {a: ['abcd'], ab: ['ef']},
+        {b: ['ancd'], a: ['xx']},
+      ],
+    };
+    assert.equal(isFileUnchanged(diff), false);
+    diff = {
+      ...createDiff(),
+      content: [{ab: ['abcd']}, {ab: ['ancd']}],
+    };
+    assert.equal(isFileUnchanged(diff), true);
+    diff = {
+      ...createDiff(),
+      content: [
+        {a: ['abcd'], ab: ['ef'], common: true},
+        {b: ['ancd'], ab: ['xx']},
+      ],
+    };
+    assert.equal(isFileUnchanged(diff), false);
+    diff = {
+      ...createDiff(),
+      content: [
+        {a: ['abcd'], ab: ['ef'], common: true},
+        {b: ['ancd'], ab: ['xx'], common: true},
+      ],
+    };
+    assert.equal(isFileUnchanged(diff), true);
+  });
 });
diff --git a/polygerrit-ui/app/models/views/change.ts b/polygerrit-ui/app/models/views/change.ts
index 3f3c76b..317e511 100644
--- a/polygerrit-ui/app/models/views/change.ts
+++ b/polygerrit-ui/app/models/views/change.ts
@@ -42,6 +42,7 @@
   repo: RepoName;
   patchNum?: RevisionPatchSetNum;
   basePatchNum?: BasePatchSetNum;
+  /** Refers to comment on COMMENTS tab in OVERVIEW. */
   commentId?: UrlEncodedCommentId;
 
   // TODO: Move properties that only apply to OVERVIEW into a submessage.
@@ -81,7 +82,6 @@
     path?: string;
     lineNum?: number;
     leftSide?: boolean;
-    commentLink?: boolean;
   };
 
   /** These properties apply to the EDIT child view only. */
diff --git a/polygerrit-ui/app/models/views/diff.ts b/polygerrit-ui/app/models/views/diff.ts
index b742c45..961c9d5 100644
--- a/polygerrit-ui/app/models/views/diff.ts
+++ b/polygerrit-ui/app/models/views/diff.ts
@@ -36,6 +36,9 @@
     suffix += state.diffView.lineNum;
   }
 
+  // TODO: Move creating of comment URLs to a separate function. We are
+  // "abusing" the `commentId` property, which should only be used for pointing
+  // to comment in the COMMENTS tab of the OVERVIEW page.
   if (state.commentId) {
     suffix = `/comment/${state.commentId}` + suffix;
   }
diff --git a/polygerrit-ui/app/utils/comment-util.ts b/polygerrit-ui/app/utils/comment-util.ts
index 8af5beb..a92f0f8 100644
--- a/polygerrit-ui/app/utils/comment-util.ts
+++ b/polygerrit-ui/app/utils/comment-util.ts
@@ -598,3 +598,17 @@
       .includes(account.email)
   );
 }
+
+export function findComment(
+  comments: {
+    [path: string]: (CommentInfo | DraftInfo)[];
+  },
+  commentId: UrlEncodedCommentId
+) {
+  if (!commentId) return undefined;
+  let comment;
+  for (const path of Object.keys(comments)) {
+    comment = comment || comments[path].find(c => c.id === commentId);
+  }
+  return comment;
+}