fix(git-ui): prevent large commit diffs from freezing the history tab

Harden commit diff loading/rendering so opening a very large commit no longer hangs
the browser tab.

Problem
- commit history diff viewer rendered every diff line as a React node
- very large commits could create thousands of nodes and lock the UI thread
- backend always returned full commit patch payloads, amplifying frontend pressure

Backend safeguards
- add `COMMIT_DIFF_CHARACTER_LIMIT` (500,000 chars) in git routes
- update GET `/api/git/commit-diff` to truncate oversized diff payloads
- include `isTruncated` flag in response for observability/future UI handling
- append truncation marker text when server-side limit is applied

Frontend safeguards
- update `GitDiffViewer` to use bounded preview rendering:
  - character cap: 200,000
  - line cap: 1,500
- move diff preprocessing into `useMemo` for stable, one-pass preview computation
- show a clear "Large diff preview" notice when truncation is active

Impact
- commit diff expansion remains responsive even for high-change commits
- UI still shows useful diff content while avoiding tab lockups
- changes apply to shared diff viewer usage and improve resilience broadly

Validation
- `node --check server/routes/git.js`
- `npm run typecheck`
- `npx eslint src/components/git-panel/view/shared/GitDiffViewer.tsx`

server/routes/git.js

@@ -7,6 +7,7 @@ import { queryClaudeSDK } from '../claude-sdk.js';
 import { spawnCursor } from '../cursor-cli.js';
 
 const router = express.Router();
+const COMMIT_DIFF_CHARACTER_LIMIT = 500_000;
 
 function spawnAsync(command, args, options = {}) {
   return new Promise((resolve, reject) => {
@@ -769,8 +770,13 @@ router.get('/commit-diff', async (req, res) => {
       'git', ['show', commit],
       { cwd: projectPath }
     );
-    
-    res.json({ diff: stdout });
+
+    const isTruncated = stdout.length > COMMIT_DIFF_CHARACTER_LIMIT;
+    const diff = isTruncated
+      ? `${stdout.slice(0, COMMIT_DIFF_CHARACTER_LIMIT)}\n\n... Diff truncated to keep the UI responsive ...`
+      : stdout;
+
+    res.json({ diff, isTruncated });
   } catch (error) {
     console.error('Git commit diff error:', error);
     res.json({ error: error.message });

src/components/git-panel/view/shared/GitDiffViewer.tsx

@@ -1,10 +1,38 @@
+import { useMemo } from 'react';
+
 type GitDiffViewerProps = {
   diff: string | null;
   isMobile: boolean;
   wrapText: boolean;
 };
 
+const PREVIEW_CHARACTER_LIMIT = 200_000;
+const PREVIEW_LINE_LIMIT = 1_500;
+
+type DiffPreview = {
+  lines: string[];
+  isCharacterTruncated: boolean;
+  isLineTruncated: boolean;
+};
+
+function buildDiffPreview(diff: string): DiffPreview {
+  const isCharacterTruncated = diff.length > PREVIEW_CHARACTER_LIMIT;
+  const previewText = isCharacterTruncated ? diff.slice(0, PREVIEW_CHARACTER_LIMIT) : diff;
+  const previewLines = previewText.split('\n');
+  const isLineTruncated = previewLines.length > PREVIEW_LINE_LIMIT;
+
+  return {
+    lines: isLineTruncated ? previewLines.slice(0, PREVIEW_LINE_LIMIT) : previewLines,
+    isCharacterTruncated,
+    isLineTruncated,
+  };
+}
+
 export default function GitDiffViewer({ diff, isMobile, wrapText }: GitDiffViewerProps) {
+  // Render a bounded preview to keep huge commit diffs from freezing the UI thread.
+  const preview = useMemo(() => buildDiffPreview(diff || ''), [diff]);
+  const isPreviewTruncated = preview.isCharacterTruncated || preview.isLineTruncated;
+
   if (!diff) {
     return (
       <div className="p-4 text-center text-sm text-muted-foreground">
@@ -35,7 +63,12 @@ export default function GitDiffViewer({ diff, isMobile, wrapText }: GitDiffViewe
 
   return (
     <div className="diff-viewer">
-      {diff.split('\n').map((line, index) => renderDiffLine(line, index))}
+      {isPreviewTruncated && (
+        <div className="mb-2 rounded-md border border-border bg-card px-3 py-2 text-xs text-muted-foreground">
+          Large diff preview: rendering is limited to keep the tab responsive.
+        </div>
+      )}
+      {preview.lines.map((line, index) => renderDiffLine(line, index))}
     </div>
   );
-}
+}