🤖 fix: reduce ProjectSidebar re-renders during streaming (#218)

## Problem

ProjectSidebar was re-rendering on every streaming delta when watching a
chat in another workspace, causing unnecessary CPU usage and UI churn.

## Solution

**Stabilize unreadStatus Map identity:**
- Modified `useUnreadTracking` to only return a new Map reference when
unread boolean values actually change
- Prevents ProjectSidebar re-renders during streaming when unread state
is stable
- Uses ref-based deep comparison to maintain Map identity across renders

**Code changes:**
```typescript
// Before: New Map created on every workspaceStates or lastReadMap change
const unreadStatus = useMemo(() => {
  const map = new Map<string, boolean>();
  // ... calculate unread status
  return map; // New identity every time
}, [workspaceStates, lastReadMap]);

// After: Map identity preserved when values unchanged
const unreadStatus = useMemo(() => {
  const next = new Map<string, boolean>();
  // ... calculate unread status
  
  // Compare with previous - only return new Map if values changed
  const prev = unreadStatusRef.current;
  if (prev.size === next.size) {
    let same = true;
    for (const [k, v] of next) {
      if (prev.get(k) !== v) {
        same = false;
        break;
      }
    }
    if (same) return prev; // Maintain identity
  }
  
  unreadStatusRef.current = next;
  return next;
}, [workspaceStates, lastReadMap]);
```

## Testing

Manual verification:
- ✅ Streaming in workspace A while viewing workspace B no longer causes
sidebar re-renders
- ✅ Unread status updates correctly on workspace switches and stream
completion
- ✅ Toggle unread function works correctly
- ✅ Drag-and-drop project reordering remains smooth

## Impact

**Fixed:**
- ProjectSidebar no longer re-renders on every streaming delta in
non-selected workspaces
- Reduced unnecessary CPU usage during streaming operations

**Not fixed (separate issue):**
- ProjectSidebar still re-renders every 3 seconds due to
`useGitStatus()` call at line 611
- Will be addressed in a follow-up PR by removing the direct hook call
and passing git status as a prop

## Notes

The comparison logic handles all edge cases correctly:
- Empty maps
- Size changes (added/removed workspaces)
- Key changes (different workspace IDs)
- Value changes (unread status flips)

_Generated with `cmux`_

Author: ammar-agent

src/contexts/GitStatusContext.tsx

@@ -31,7 +31,7 @@ interface GitStatusProviderProps {
  * - Max 5 concurrent git status checks to prevent bash process explosion
  */
 // Configuration - enabled by default, no env variables needed
-const GIT_STATUS_INTERVAL_MS = 3000; // 3 seconds
+const GIT_STATUS_INTERVAL_MS = 3000; // 3 seconds - interactive updates
 const MAX_CONCURRENT_GIT_OPS = 5;
 
 // Fetch configuration - aggressive intervals for fresh data

src/hooks/useUnreadTracking.ts

@@ -73,13 +73,14 @@ export function useUnreadTracking(
   }, [selectedWorkspace?.workspaceId, workspaceStates, markAsRead]);
 
   // Calculate unread status for all workspaces
+  const unreadStatusRef = useRef<Map<string, boolean>>(new Map());
   const unreadStatus = useMemo(() => {
-    const result = new Map<string, boolean>();
+    const next = new Map<string, boolean>();
 
     for (const [workspaceId, state] of workspaceStates) {
       // Streaming workspaces are never unread
       if (state.canInterrupt) {
-        result.set(workspaceId, false);
+        next.set(workspaceId, false);
         continue;
       }
 
@@ -92,10 +93,26 @@ export function useUnreadTracking(
           msg.type !== "user" && msg.type !== "history-hidden" && (msg.timestamp ?? 0) > lastRead
       );
 
-      result.set(workspaceId, hasUnread);
+      next.set(workspaceId, hasUnread);
     }
 
-    return result;
+    // Return previous Map reference if nothing actually changed to keep identity stable
+    const prev = unreadStatusRef.current;
+    if (prev.size === next.size) {
+      let same = true;
+      for (const [k, v] of next) {
+        if (prev.get(k) !== v) {
+          same = false;
+          break;
+        }
+      }
+      if (same) {
+        return prev;
+      }
+    }
+
+    unreadStatusRef.current = next;
+    return next;
   }, [workspaceStates, lastReadMap]);
 
   // Manual toggle function for clicking the indicator