Reschedule engine frame if it arrives in the middle of warm-up (#72115)

* Reschedule engine frame if it arrives in the middle of warm-up

* user post-frame callback instead

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

@@ -954,20 +954,45 @@ mixin SchedulerBinding on BindingBase {
 
   int _debugFrameNumber = 0;
   String? _debugBanner;
-  bool _ignoreNextEngineDrawFrame = false;
+
+  // Whether the current engine frame needs to be postponed till after the
+  // warm-up frame.
+  //
+  // Engine may begin a frame in the middle of the warm-up frame because the
+  // warm-up frame is scheduled by timers while the engine frame is scheduled
+  // by platform specific frame scheduler (e.g. `requestAnimationFrame` on the
+  // web). When this happens, we let the warm-up frame finish, and postpone the
+  // engine frame.
+  bool _rescheduleAfterWarmUpFrame = false;
 
   void _handleBeginFrame(Duration rawTimeStamp) {
     if (_warmUpFrame) {
-      assert(!_ignoreNextEngineDrawFrame);
-      _ignoreNextEngineDrawFrame = true;
+      // "begin frame" and "draw frame" must strictly alternate. Therefore
+      // _rescheduleAfterWarmUpFrame cannot possibly be true here as it is
+      // reset by _handleDrawFrame.
+      assert(!_rescheduleAfterWarmUpFrame);
+      _rescheduleAfterWarmUpFrame = true;
       return;
     }
     handleBeginFrame(rawTimeStamp);
   }
 
   void _handleDrawFrame() {
-    if (_ignoreNextEngineDrawFrame) {
-      _ignoreNextEngineDrawFrame = false;
+    if (_rescheduleAfterWarmUpFrame) {
+      _rescheduleAfterWarmUpFrame = false;
+      // Reschedule in a post-frame callback to allow the draw-frame phase of
+      // the warm-up frame to finish.
+      addPostFrameCallback((Duration timeStamp) {
+        // Force an engine frame.
+        //
+        // We need to reset _hasScheduledFrame here because we cancelled the
+        // original engine frame, and therefore did not run handleBeginFrame
+        // who is responsible for resetting it. So if a frame callback set this
+        // to true in the "begin frame" part of the warm-up frame, it will
+        // still be true here and cause us to skip scheduling an engine frame.
+        _hasScheduledFrame = false;
+        scheduleFrame();
+      });
       return;
     }
     handleDrawFrame();

packages/flutter/test/scheduler/scheduler_test.dart

@@ -129,6 +129,11 @@ void main() {
     // events are locked.
     expect(timerQueueTasks.length, 2);
     expect(taskExecuted, false);
+
+    // Run the timers so that the scheduler is no longer in warm-up state.
+    for (final VoidCallback timer in timerQueueTasks) {
+      timer();
+    }
   });
 
   test('Flutter.Frame event fired', () async {
@@ -165,6 +170,9 @@ void main() {
   });
 
   test('currentSystemFrameTimeStamp is the raw timestamp', () {
+    // Undo epoch set by previous tests.
+    scheduler.resetEpoch();
+
     late Duration lastTimeStamp;
     late Duration lastSystemTimeStamp;
 
@@ -195,6 +203,40 @@ void main() {
     expect(lastTimeStamp, const Duration(seconds: 3)); // 2s + (8 - 6)s / 2
     expect(lastSystemTimeStamp, const Duration(seconds: 8));
   });
+
+  test('Animation frame scheduled in the middle of the warm-up frame', () {
+    expect(scheduler.schedulerPhase, SchedulerPhase.idle);
+    final List<VoidCallback> timers = <VoidCallback>[];
+    final ZoneSpecification timerInterceptor = ZoneSpecification(
+      createTimer: (Zone self, ZoneDelegate parent, Zone zone, Duration duration, void Function() callback) {
+        timers.add(callback);
+        return DummyTimer();
+      },
+    );
+
+    // Schedule a warm-up frame.
+    // Expect two timers, one for begin frame, and one for draw frame.
+    runZoned<void>(scheduler.scheduleWarmUpFrame, zoneSpecification: timerInterceptor);
+    expect(timers.length, 2);
+    final VoidCallback warmUpBeginFrame = timers.first;
+    final VoidCallback warmUpDrawFrame = timers.last;
+    timers.clear();
+
+    warmUpBeginFrame();
+
+    // Simulate an animation frame firing between warm-up begin frame and warm-up draw frame.
+    // Expect a timer that reschedules the frame.
+    expect(scheduler.hasScheduledFrame, isFalse);
+    window.onBeginFrame!(Duration.zero);
+    expect(scheduler.hasScheduledFrame, isFalse);
+    window.onDrawFrame!();
+    expect(scheduler.hasScheduledFrame, isFalse);
+
+    // The draw frame part of the warm-up frame will run the post-frame
+    // callback that reschedules the engine frame.
+    warmUpDrawFrame();
+    expect(scheduler.hasScheduledFrame, isTrue);
+  });
 }
 
 class DummyTimer implements Timer {