fix(ui): prevent memory leak in diff view with many modified files

- Add LRU eviction to diffCache (max 30 entries / 20MB)
- Single-file mode: render only selected diff, not all hidden ones
- Stacked mode: collapse files beyond first 10 by default
- Limit DiffWorkerProvider warmup to 10 files
- Reduce worker pool size (4 to 2) and AST cache (200 to 50)

packages/ui/src/components/views/DiffView.tsx

@@ -29,6 +29,9 @@ import { useDeviceInfo } from '@/lib/device';
 const SIDE_BY_SIDE_MIN_WIDTH = 1100;
 const DIFF_REQUEST_TIMEOUT_MS = 15000;
 
+// Memory optimization: limit concurrent expanded diffs in stacked view
+const STACKED_VIEW_MAX_EXPANDED_DIFFS = 10;
+
 type FileEntry = GitStatus['files'][number] & {
     insertions: number;
     deletions: number;
@@ -453,7 +456,7 @@ const InlineDiffViewer = React.memo<InlineDiffViewerProps>(({
     );
 });
 
-// Single diff viewer instance - stays mounted
+// Single diff viewer instance
 interface SingleDiffViewerProps {
     filePath: string;
     diff: DiffData;
@@ -474,6 +477,11 @@ const SingleDiffViewer = React.memo<SingleDiffViewerProps>(({
         [filePath]
     );
 
+    // Don't render if not visible (memory optimization)
+    if (!isVisible) {
+        return null;
+    }
+
     // Check if this is an image file
     if (isImageFile(filePath)) {
         return (
@@ -486,23 +494,6 @@ const SingleDiffViewer = React.memo<SingleDiffViewerProps>(({
         );
     }
 
-    // Use display:none for hidden diffs to exclude from layout calculations during resize
-    // This is faster for resize than visibility:hidden which keeps elements in layout flow
-    if (!isVisible) {
-        return (
-            <div className="absolute inset-0 hidden">
-                <PierreDiffViewer
-                    original={diff.original}
-                    modified={diff.modified}
-                    language={language}
-                    fileName={filePath}
-                    renderSideBySide={renderSideBySide}
-                    wrapLines={wrapLines}
-                />
-            </div>
-        );
-    }
-
     return (
         <div className="absolute inset-0" style={{ contain: 'size layout' }}>
             <PierreDiffViewer
@@ -565,6 +556,8 @@ interface MultiFileDiffEntryProps {
     isSelected: boolean;
     onSelect: (path: string) => void;
     registerSectionRef: (path: string, node: HTMLDivElement | null) => void;
+    /** Start collapsed to reduce memory with many files */
+    defaultCollapsed?: boolean;
 }
 
 const MultiFileDiffEntry = React.memo<MultiFileDiffEntryProps>(({
@@ -576,6 +569,7 @@ const MultiFileDiffEntry = React.memo<MultiFileDiffEntryProps>(({
     isSelected,
     onSelect,
     registerSectionRef,
+    defaultCollapsed = false,
 }) => {
     const { git } = useRuntimeAPIs();
     const cachedDiff = useGitStore(
@@ -586,7 +580,7 @@ const MultiFileDiffEntry = React.memo<MultiFileDiffEntryProps>(({
     const setDiff = useGitStore((state) => state.setDiff);
     const setDiffFileLayout = useUIStore((state) => state.setDiffFileLayout);
 
-    const [isExpanded, setIsExpanded] = React.useState(true);
+    const [isExpanded, setIsExpanded] = React.useState(!defaultCollapsed);
     const [hasBeenVisible, setHasBeenVisible] = React.useState(false);
     const [diffRetryNonce, setDiffRetryNonce] = React.useState(0);
     const [diffLoadError, setDiffLoadError] = React.useState<string | null>(null);
@@ -947,6 +941,7 @@ export const DiffView: React.FC = () => {
 
     const handleSelectFileAndScroll = React.useCallback((value: string) => {
         setSelectedFile(value);
+
         if (isStackedView && !scrollToFile(value)) {
             pendingScrollTargetRef.current = value;
         }
@@ -1040,20 +1035,20 @@ export const DiffView: React.FC = () => {
         };
     }, [effectiveDirectory, isStackedView, selectedFile, selectedCachedDiff, git, setDiff, diffRetryNonce]);
 
-    // Render all diff viewers - they stay mounted
-    const renderAllDiffViewers = () => {
-        if (!effectiveDirectory || changedFiles.length === 0) return null;
+    // Render only the selected diff viewer to prevent memory bloat with many files
+    const renderSelectedDiffViewer = () => {
+        if (!effectiveDirectory || !selectedFile) return null;
 
-        return changedFiles.map((file) => (
+        return (
             <DiffViewerEntry
-                key={file.path}
+                key={selectedFile}
                 directory={effectiveDirectory}
-                filePath={file.path}
-                isVisible={file.path === selectedFile}
+                filePath={selectedFile}
+                isVisible={true}
                 renderSideBySide={renderSideBySide}
                 wrapLines={diffWrapLines}
             />
-        ));
+        );
     };
 
     const renderStackedDiffView = () => {
@@ -1081,7 +1076,7 @@ export const DiffView: React.FC = () => {
                     disableHorizontal
                 >
                     <div className="flex flex-col gap-3">
-                        {changedFiles.map((file) => (
+                        {changedFiles.map((file, index) => (
                             <MultiFileDiffEntry
                                 key={file.path}
                                 directory={effectiveDirectory}
@@ -1092,6 +1087,7 @@ export const DiffView: React.FC = () => {
                                 isSelected={file.path === selectedFile}
                                 onSelect={handleSelectFile}
                                 registerSectionRef={registerSectionRef}
+                                defaultCollapsed={index >= STACKED_VIEW_MAX_EXPANDED_DIFFS}
                             />
                         ))}
                     </div>
@@ -1141,7 +1137,7 @@ export const DiffView: React.FC = () => {
 
         return (
             <div className="flex flex-1 min-h-0 overflow-hidden px-3 py-3 relative">
-                {renderAllDiffViewers()}
+                {renderSelectedDiffViewer()}
                 {isCurrentFileLoading && !hasCurrentDiff && (
                     <div className="absolute inset-0 flex items-center justify-center gap-2 text-sm text-muted-foreground">
                         {diffLoadError ? (