Fix WidgetsBinding.firstFrameRasterized not completing

[jiahaog]

Description

The completer backing WidgetsBinding.firstFrameRasterized never completes when deferFirstFrame is used.

Please see jiahaog/repro_flutter_first_frame@ed49ba4 for a small reproduction.

This is because firstFrameCallback is removed here when the current frame is a deferred frame

flutter/packages/flutter/lib/src/widgets/binding.dart

Line 880 in bd6ccb6

SchedulerBinding.instance.removeTimingsCallback(firstFrameCallback);

but _needToReportFirstFrame is reset back to true. As a result, the next iteration of drawFrame for the frame that should be sent to the engine will never trigger the conditional block that reports the frames.

flutter/packages/flutter/lib/src/widgets/binding.dart

Lines 843 to 860 in bd6ccb6

	if (_needToReportFirstFrame) {
	assert(!_firstFrameCompleter.isCompleted);
	firstFrameCallback = (List<FrameTiming> timings) {
	assert(sendFramesToEngine);
	if (!kReleaseMode) {
	developer.Timeline.instantSync('Rasterized first useful frame');
	developer.postEvent('Flutter.FirstFrame', <String, dynamic>{});
	}
	SchedulerBinding.instance.removeTimingsCallback(firstFrameCallback);
	firstFrameCallback = null;
	_firstFrameCompleter.complete();
	};
	// Callback is only invoked when [Window.render] is called. When
	// [sendFramesToEngine] is set to false during the frame, it will not
	// be called and we need to remove the callback (see below).
	SchedulerBinding.instance.addTimingsCallback(firstFrameCallback);
	}

This affects driver tests as well when the FlutterDriver.waitUntilFirstFrameRasterized is used. (Internal bug b/155033896)

Related Issues

#45135

Tests

I added the following tests:

• Add a test where deferred frames will trigger the first frame rasterized flag.

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[goderbauer]

Good catch! The fix looks right. In order to accept the PR we will need a test, though. Can you please add one? Thanks.

[jiahaog]

Please let me know if there's a better way to do this, as there doesn't seem to be an engine in the tests that actually calls the callbacks.

[jiahaog]

@goderbauer added tests, please take a look!

[goderbauer]

LGTM after comments are resolved.

[goderbauer]

LGTM