fix(desktop): throttle hidden renderers

apps/desktop/src/app/session/hooks/use-message-stream/delta-flush.test.tsx

@@ -90,6 +90,42 @@ describe('useMessageStream delta flush scheduling', () => {
     expect(assistantText()).toBe('still streaming')
   })
 
+  it('flushes queued text immediately when a hidden window becomes visible', () => {
+    vi.mocked(performance.now).mockReturnValue(0)
+    mountStream()
+
+    act(() => appendAssistantDelta!(SID, 'caught up on focus'))
+    expect(assistantText()).toBe('')
+    expect(vi.getTimerCount()).toBe(1)
+
+    Object.defineProperty(globalThis.document, 'visibilityState', {
+      configurable: true,
+      value: 'visible'
+    })
+
+    act(() => globalThis.document.dispatchEvent(new Event('visibilitychange')))
+
+    expect(assistantText()).toBe('caught up on focus')
+    expect(vi.getTimerCount()).toBe(0)
+  })
+
+  it('flushes queued text on focus when visibility remains visible', () => {
+    vi.mocked(performance.now).mockReturnValue(0)
+    Object.defineProperty(globalThis.document, 'visibilityState', {
+      configurable: true,
+      value: 'visible'
+    })
+    mountStream()
+
+    act(() => appendAssistantDelta!(SID, 'focused without visibility change'))
+    expect(assistantText()).toBe('')
+    expect(vi.getTimerCount()).toBe(1)
+
+    act(() => globalThis.window.dispatchEvent(new Event('focus')))
+
+    expect(assistantText()).toBe('focused without visibility change')
+  })
+
   it('cancels the pending timer on unmount and flushes exactly once', async () => {
     vi.mocked(performance.now).mockReturnValue(0)
     mountStream()

apps/desktop/src/app/session/hooks/use-message-stream/index.ts

@@ -316,6 +316,34 @@ export function useMessageStream({
     [flushQueuedDeltas]
   )
 
+  // Page Visibility does not report every Windows/Linux focus transition.
+  // Flush queued deltas on both signals so returning to a chat cannot leave a
+  // completed chunk waiting for the next throttled timer.
+  useEffect(() => {
+    const flushPendingDeltas = () => {
+      if (flushHandleRef.current !== null) {
+        window.clearTimeout(flushHandleRef.current)
+        flushHandleRef.current = null
+      }
+
+      flushQueuedDeltas()
+    }
+
+    const flushWhenVisible = () => {
+      if (document.visibilityState === 'visible') {
+        flushPendingDeltas()
+      }
+    }
+
+    document.addEventListener('visibilitychange', flushWhenVisible)
+    window.addEventListener('focus', flushPendingDeltas)
+
+    return () => {
+      document.removeEventListener('visibilitychange', flushWhenVisible)
+      window.removeEventListener('focus', flushPendingDeltas)
+    }
+  }, [flushQueuedDeltas])
+
   const appendAssistantDelta = useCallback(
     (sessionId: string, delta: string) => {
       if (!delta) {