Fix HTML preview: always-visible buttons and multi-pane group selection

- Make preview buttons always visible (not hover-reveal) for touch support
- Fix event propagation so clicking preview doesn't toggle sections
- Use combined-diff tab's group for sourceGroupId in multi-pane layouts
- Add accessibility label to open-section button
- Support untracked, renamed, and uppercase HTML file extensions

src/renderer/src/components/editor/CombinedDiffViewer.tsx

@@ -1299,14 +1299,24 @@ export default function CombinedDiffViewer({
       ) {
         return
       }
+      // Why: use this combined-diff tab's group, not worktree activeGroupId —
+      // in a multi-pane layout the active group may be a different split.
+      const state = useAppStore.getState()
+      const sourceGroupId =
+        (state.unifiedTabsByWorktree[file.worktreeId] ?? []).find(
+          (tab) =>
+            tab.entityId === file.id && (tab.contentType === 'diff' || tab.contentType === 'editor')
+        )?.groupId ??
+        activeGroupId ??
+        null
       openFilePreviewToSide({
         language: detectLanguage(section.path),
         filePath: joinPath(file.filePath, section.path),
         worktreeId: file.worktreeId,
-        sourceGroupId: activeGroupId ?? null
+        sourceGroupId
       })
     },
-    [activeGroupId, file.filePath, file.worktreeId, isCommitMode]
+    [activeGroupId, file.filePath, file.id, file.worktreeId, isCommitMode]
   )
 
   const handleSectionSave = useCallback(
@@ -2055,7 +2065,7 @@ export default function CombinedDiffViewer({
                             status: section.status,
                             isCommitSurface: isCommitMode
                           })
-                            ? (previewSection) => openSectionPreview(previewSection)
+                            ? openSectionPreview
                             : undefined
                         }
                         setSectionHeights={setSectionHeights}

src/renderer/src/components/editor/DiffSectionHeader.tsx

@@ -76,10 +76,14 @@ export function DiffSectionHeader({
         {onOpenPreview != null && (
           <button
             type="button"
-            // Why: always visible (unlike other hover-reveal section actions) so
-            // HTML preview matches the single-file editor header affordance.
+            // Why: always visible (not hover-reveal) so HTML preview matches the
+            // single-file editor header and stays usable without hover (touch).
             className="p-0.5 rounded text-muted-foreground hover:text-foreground transition-colors"
-            onClick={onOpenPreview}
+            onClick={(event) => {
+              // Why: header owns stopPropagation so callers cannot forget and toggle the section.
+              event.stopPropagation()
+              onOpenPreview(event)
+            }}
             title={translate(
               'auto.components.editor.EditorPanelHeader.fb8331694e',
               'Open Preview to the Side'
@@ -96,8 +100,12 @@ export function DiffSectionHeader({
           // Why: always visible so open-file matches Eye preview and works without hover (touch).
           type="button"
           className="p-0.5 rounded text-muted-foreground hover:text-foreground transition-colors"
-          onClick={onOpenSection}
+          onClick={(event) => {
+            event.stopPropagation()
+            onOpenSection(event)
+          }}
           title={openSectionTitle}
+          aria-label={openSectionTitle}
         >
           <ExternalLink className="size-3.5" />
         </button>

src/renderer/src/components/editor/DiffSectionItem.tsx

@@ -346,8 +346,7 @@ export function DiffSectionItem({
         openSectionTitle={openSectionTitle}
         onOpenPreview={
           onOpenPreview
-            ? (event) => {
-                event.stopPropagation()
+            ? () => {
                 onOpenPreview(section, index)
               }
             : undefined

src/renderer/src/components/editor/diff-section-preview.test.ts

@@ -22,6 +22,33 @@ describe('canOpenDiffSectionPreviewToSide', () => {
     ).toBe(true)
   })
 
+  it('enables untracked and renamed HTML files still present on disk', () => {
+    expect(
+      canOpenDiffSectionPreviewToSide({
+        path: 'scratch.html',
+        status: 'untracked',
+        isCommitSurface: false
+      })
+    ).toBe(true)
+    expect(
+      canOpenDiffSectionPreviewToSide({
+        path: 'renamed.html',
+        status: 'renamed',
+        isCommitSurface: false
+      })
+    ).toBe(true)
+  })
+
+  it('enables uppercase HTML extensions', () => {
+    expect(
+      canOpenDiffSectionPreviewToSide({
+        path: 'Docs/DEMO.HTML',
+        status: 'modified',
+        isCommitSurface: false
+      })
+    ).toBe(true)
+  })
+
   it('disables deleted HTML files', () => {
     expect(
       canOpenDiffSectionPreviewToSide({