Fix focus being on the LOST diff row for image diffs

This was found using screenshot testing and looks like this:
https://imgur.com/a/ZS05wiR

The `buildSectionElement()` override is a pattern that we also use
for the binary diff builder.

Release-Notes: skip
Change-Id: I4493ea1b2280227c791c2704c112ec57d088fa71
(cherry picked from commit 36fa69d8f072f6e5c1ef1c25debea7ccb7542137)

1 files changed, 9 insertions, 0 deletions

diff --git a/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-image.ts b/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-image.ts
index 3bffe08bc0..1f7ffd38f8 100644
--- a/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-image.ts
+++ b/polygerrit-ui/app/embed/diff/gr-diff-builder/gr-diff-builder-image.ts
@@ -10,6 +10,8 @@ import '../gr-diff-image-viewer/gr-image-viewer';
 import {html, LitElement, nothing} from 'lit';
 import {customElement, property, query, state} from 'lit/decorators.js';
 import {GrDiffBuilder} from './gr-diff-builder';
+import {createElementDiff} from '../gr-diff/gr-diff-utils';
+import {GrDiffGroup} from '../gr-diff/gr-diff-group';
 
 // MIME types for images we allow showing. Do not include SVG, it can contain
 // arbitrary JavaScript.
@@ -28,6 +30,13 @@ export class GrDiffBuilderImage extends GrDiffBuilder {
     super(diff, prefs, outputEl, [], renderPrefs);
   }
 
+  override buildSectionElement(group: GrDiffGroup): HTMLElement {
+    const section = createElementDiff('tbody');
+    // Do not create a diff row for 'LOST'.
+    if (group.lines[0].beforeNumber !== 'FILE') return section;
+    return super.buildSectionElement(group);
+  }
+
   public renderImageDiff() {
     const imageDiff = this.useNewImageDiffUi
       ? this.createImageDiffNew()