test(mobile): assert migrated screencast frames reach the original listener

The shape-only assertion passed on a no-op callback, which is the dropped-frame
bug this migration path exists to fix. Also pin what a superseded frame's late
onLoad promotes.

mobile/src/browser/mobile-browser-frameless-stream.test.tsx

@@ -294,6 +294,57 @@ describe('MobileBrowserPane with a stream that reports ready but sends no frames
     expect(activeSubscriptionCount()).toBe(1)
   })
 
+  // Why: the load handler keys on (layer, decoderEpoch) and a B->C replacement reuses both,
+  // so a late B onLoad does promote. Pin what that promotion actually lands on.
+  it('promotes the newest frame when a superseded frame reports its load late', async () => {
+    vi.useFakeTimers()
+    vi.setSystemTime(1_000)
+    const { renderer, stream, nativeMocks } = await renderPane()
+
+    act(() => {
+      stream.onBinaryFrame?.(makeFrame('visible', 1))
+    })
+    const layers = getFrameLayers(renderer, nativeMocks)
+
+    act(() => {
+      stream.onBinaryFrame?.(makeFrame('superseded', 2))
+      vi.advanceTimersByTime(100)
+    })
+    expect(latestNativeImageUri(layers[1].imageNative)).toBe(frameUri('superseded'))
+
+    // The final frame replaces the hidden layer's source without a new transition.
+    act(() => {
+      stream.onBinaryFrame?.(makeFrame('final', 3))
+      vi.advanceTimersByTime(100)
+    })
+    expect(latestNativeImageUri(layers[1].imageNative)).toBe(frameUri('final'))
+
+    for (const layer of layers) {
+      layer.wrapperNative.setNativeProps.mockClear()
+    }
+
+    // The superseded frame's load arrives after its source was replaced.
+    act(() => layers[1].image.props.onLoad())
+
+    // The promoted layer is already pointed at the newest frame, not the superseded one,
+    // and the layer it replaces held a strictly older frame.
+    expect(latestNativeImageUri(layers[1].imageNative)).toBe(frameUri('final'))
+    expect(layers[0].wrapperNative.setNativeProps).toHaveBeenLastCalledWith({
+      style: { opacity: 0 }
+    })
+    expect(layers[1].wrapperNative.setNativeProps).toHaveBeenLastCalledWith({
+      style: { opacity: 1 }
+    })
+
+    // The newest frame's own load is then a no-op: the transition is already settled.
+    for (const layer of layers) {
+      layer.wrapperNative.setNativeProps.mockClear()
+    }
+    act(() => layers[1].image.props.onLoad())
+    expect(layers[0].wrapperNative.setNativeProps).not.toHaveBeenCalled()
+    expect(layers[1].wrapperNative.setNativeProps).not.toHaveBeenCalled()
+  })
+
   it('does no rendering or native work on a healthy connected boundary', async () => {
     const {
       renderer,

mobile/src/transport/stable-logical-rpc-client.test.ts

@@ -128,8 +128,9 @@ describe('stable logical RPC client', () => {
       const replacement = new FakeSession('connected')
       const client = createStableLogicalRpcClient(oldSession, fromPath)
       const states: ConnectionState[] = []
+      const onBinaryFrame = vi.fn()
       client.subscribe('browser.screencast', { worktree: 'id:wt-1', page: 'page-1' }, vi.fn(), {
-        onBinaryFrame: vi.fn()
+        onBinaryFrame
       })
       client.onStateChange((state) => states.push(state))
 
@@ -142,6 +143,15 @@ describe('stable logical RPC client', () => {
         expect.any(Function),
         { onBinaryFrame: expect.any(Function) }
       )
+      // Why invoke it: asserting the shape alone passes on a no-op, which is exactly the
+      // dropped-frame bug this migration path is meant to fix.
+      const replayedOpts = replacement.subscribe.mock.calls[0]![3] as {
+        onBinaryFrame?: (frame: unknown) => void
+      }
+      const frame = { opcode: 1, seq: 7 }
+      replayedOpts.onBinaryFrame?.(frame)
+      expect(onBinaryFrame).toHaveBeenCalledOnce()
+      expect(onBinaryFrame).toHaveBeenCalledWith(frame)
       expect(oldSession.close).toHaveBeenCalledOnce()
       expect(states).toEqual(['connected'])
       expect(client.getActivePath()).toBe(toPath)