I'm wondering what first_present_called_ represents since it doesn't seem to care about whether DoPresent() actually did the presentation.

Will OnGetLayout() return the correct value if DoPresent wasn't called yet (either because this thread didn't reach DoPresent() or because present_credits_ == 0 on this call)?

Flutter animator waits for a Vsync signal from Flatland before starting to render a frame, see Animator::RequestFrame(). Flatland returns Vsync signal to the client through OnNextFrameBegin(), but only after they send their first Present(). |first_present_called_| was used to fake the first vsync signal in AwaitVsync(). We then follow Present()-OnNextFrame() sequence for the render loop.

Flatland's OnGetLayout() returns values to Flutter on a separate protocol and platform thread. Ideally, the clients should wait for it before their first Present() so they know how big their content should be. Flutter does that and even skips render if size isn't there yet in Engine::Render(). However, it can take a couple AwaitVsync calls to have the size, so we fake vsync signals until the first Present().

All Flatland clients start with 1 present credit, so we know the first Present goes through.

Is acquiring and releasing this lock a second time on every frame expensive? It seems like we're adding additional work to every frame to handle a case that's only true for the first couple of vsyncs and never afterwards.

If this is cheap ignore this comment.

Good call! I am changing so that we keep the lock. I was trying to handle the case that FlatlandConnectionTest.PresentCreditExhaustion exercises where FireCallbackCallback runs Present(). But it is not applicable in production code as we have a thread jump. AwaitVsync is always on UI thread and Present() is on raster thread. I am modifying the test.