fix(review): remount DiffViewer on mode switch to invalidate stale content

Switching between Branch diff and PR Diff for the same file and base
didn't trigger a file-content refetch — the useEffect deps (filePath,
oldPath, reviewBase) were all unchanged. But the two modes compute
different "old" content (base tip vs merge-base ancestor), so Pierre
could mismatch or show wrong expansion context.

Fix: expose activeDiffBase through ReviewState and include it in the
ReviewDiffPanel key. Mode changes now remount DiffViewer, which clears
the cached content and fetches fresh. Same trade-off as the existing
reviewBase-keyed remount (scroll position resets on mode switch).

For provenance purposes, this commit was AI assisted.

Author: backnotprop

packages/review-editor/App.tsx

@@ -1128,6 +1128,7 @@ const ReviewApp: React.FC = () => {
       (activeDiffBase === 'branch' || activeDiffBase === 'merge-base')
         ? committedBase ?? undefined
         : undefined,
+    activeDiffBase,
     feedbackDiffContext,
     allAnnotations,
     externalAnnotations,

packages/review-editor/dock/ReviewStateContext.tsx

@@ -29,6 +29,10 @@ export interface ReviewState {
   fontSize?: string;
   /** User-selected base branch; feeds the `base` query param on file-content fetches. */
   reviewBase?: string;
+  /** Active diff mode (e.g. "branch", "merge-base", "uncommitted"). Used as
+   *  part of the DiffViewer remount key so mode switches invalidate cached
+   *  file content — branch and merge-base compute different "old" sides. */
+  activeDiffBase?: string;
   /** Diff context baked into exported feedback so downstream panels (agent job
    * detail, etc.) produce the same markdown the main feedback path sends. */
   feedbackDiffContext?: FeedbackDiffContext;

packages/review-editor/dock/panels/ReviewDiffPanel.tsx

@@ -56,7 +56,7 @@ export const ReviewDiffPanel: React.FC<IDockviewPanelProps> = (props) => {
   // patch, and Pierre briefly reconciles old-patch + new-content → "trailing
   // context mismatch" warnings in the console.
   return (
-    <div key={`${file.path}:${state.reviewBase ?? ''}`} className="h-full relative">
+    <div key={`${file.path}:${state.reviewBase ?? ''}:${state.activeDiffBase ?? ''}`} className="h-full relative">
       <DiffViewer
         patch={file.patch}
         filePath={file.path}