🤖 fix: prevent UI freeze during reasoning stream by bypassing useDeferredValue (#1158)

## Summary

Fixes a bug where stream updates stopped showing in the UI during
thinking blocks, while the token counter continued showing activity.

## Root Cause

The `useDeferredValue` hook can defer rendering indefinitely when React
keeps receiving new work (rapid streaming deltas). The previous logic
only showed immediate messages when the message array length changed:

```tsx
const deferredMessages =
  transformedMessages.length !== deferredTransformedMessages.length
    ? transformedMessages
    : deferredTransformedMessages;
```

During reasoning block streaming, the array length stays constant - only
the content changes within existing reasoning messages. This caused
React to keep deferring indefinitely.

## Symptoms

- Token counter kept updating (reads aggregator directly during render,
bypassing React state)
- Message UI appeared frozen (useDeferredValue kept deferring updates)
- Reload fixed it (cleared the deferral queue)

## Fix

Added a `hasActiveStream` check that bypasses deferred rendering when
any message has `isStreaming=true`, ensuring real-time UI updates during
streaming while still benefiting from deferred rendering for idle
scrollback:

```tsx
const hasActiveStream = transformedMessages.some((m) => "isStreaming" in m && m.isStreaming);
const deferredMessages =
  hasActiveStream || transformedMessages.length !== deferredTransformedMessages.length
    ? transformedMessages
    : deferredTransformedMessages;
```

---
_Generated with `mux` • Model: `anthropic:claude-opus-4-5` • Thinking:
`medium`_

Author: ammar-agent

src/browser/components/AIView.tsx

@@ -173,11 +173,14 @@ const AIViewInner: React.FC<AIViewProps> = ({
   const transformedMessages = useMemo(() => mergeConsecutiveStreamErrors(messages), [messages]);
   const deferredTransformedMessages = useDeferredValue(transformedMessages);
 
-  // CRITICAL: When message count changes (new message sent/received), show immediately.
-  // Only defer content changes within existing messages (streaming deltas).
-  // This ensures user messages appear instantly while keeping streaming performant.
+  // CRITICAL: Show immediate messages when streaming or when message count changes.
+  // useDeferredValue can defer indefinitely if React keeps getting new work (rapid deltas).
+  // During active streaming (reasoning, text), we MUST show immediate updates or the UI
+  // appears frozen while only the token counter updates (reads aggregator directly).
+  // Only use deferred messages when the stream is idle and no content is changing.
+  const hasActiveStream = transformedMessages.some((m) => "isStreaming" in m && m.isStreaming);
   const deferredMessages =
-    transformedMessages.length !== deferredTransformedMessages.length
+    hasActiveStream || transformedMessages.length !== deferredTransformedMessages.length
       ? transformedMessages
       : deferredTransformedMessages;