testKeyboardAnimationWillWaitUIThreadVsync posted a sleep(1) task to the UI task runner and asserted that the keyboard animation callback did not fire until at least that delay had elapsed. Whether or not the callback actually observed the delay depended on thread scheduling, which made the test flaky.

The property the old test was attempting to verify, that the callback does not run until in-flight UI-thread work completes, is not implemented by this code, so there is nothing here for a test to protect. This is guaranteed by the run-loop itself. The vsync client registers its CADisplayLink on the UI task runner's run loop (see setUpKeyboardAnimationVsyncClient in FlutterKeyboardInsetManager and VSyncClient.init), and a run loop services one source at a time, so the display-link callback can't possibly run while the UI thread is busy processing a begin-frame event. Asserting that ordering means asserting CFRunLoop and CADisplayLink scheduling semantics, which is not only outside our control but inherently racy/fragile, which causes flakes.

What this code does enforce is that delivery is asynchronous. The callback is wrapped in dispatch_async(dispatch_get_main_queue()) rather than invoked inline from -onDisplayLink:. This updates the test to trigger the display link directly, asserts the callback has not run synchronously, then waits on an XCTestExpectation and asserts that it's eventually delivered on the main queue.

The view controller is now built with FlutterEnginePartialMock, whose uiTaskRunner runs on the test's own message loop, so the vsync client's display-link registration and invalidation execute deterministically on the current thread. I've also renamed the test to
testKeyboardAnimationCallbackIsDeliveredAsynchronously to match what it actually verifies.

As mentioned above, this intentionally drops coverage of the wait-for-UI-thread behavior originally added for
#120555. As noted, that's already guaranteed by the run-loop and unrelated to our own logic.

Issue: #189352

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