ReportTiming callback should record the sendFrameToEngine when it was scheduled (#144212)

Fixes https://github.com/flutter/flutter/issues/144261

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

@@ -991,11 +991,12 @@ mixin WidgetsBinding on BindingBase, ServicesBinding, SchedulerBinding, GestureB
     }());
 
     TimingsCallback? firstFrameCallback;
+    bool debugFrameWasSentToEngine = false;
     if (_needToReportFirstFrame) {
       assert(!_firstFrameCompleter.isCompleted);
 
       firstFrameCallback = (List<FrameTiming> timings) {
-        assert(sendFramesToEngine);
+        assert(debugFrameWasSentToEngine);
         if (!kReleaseMode) {
           // Change the current user tag back to the default tag. At this point,
           // the user tag should be set to "AppStartUp" (originally set in the
@@ -1020,6 +1021,10 @@ mixin WidgetsBinding on BindingBase, ServicesBinding, SchedulerBinding, GestureB
         buildOwner!.buildScope(rootElement!);
       }
       super.drawFrame();
+      assert(() {
+        debugFrameWasSentToEngine = sendFramesToEngine;
+        return true;
+      }());
       buildOwner!.finalizeTree();
     } finally {
       assert(() {

packages/flutter/test/widgets/binding_live_test.dart

@@ -0,0 +1,64 @@
+// Copyright 2014 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:flutter/foundation.dart';
+import 'package:flutter/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+
+// This file is for tests for WidgetsBinding that require a `LiveTestWidgetsFlutterBinding`.
+void main() {
+  LiveTestWidgetsFlutterBinding();
+  testWidgets('ReportTiming callback records the sendFramesToEngine when it was scheduled', (WidgetTester tester) async {
+    // Addresses https://github.com/flutter/flutter/issues/144261
+    // This test needs LiveTestWidgetsFlutterBinding for multiple reasons.
+    //
+    // First, this was the environment that this bug was discovered.
+    //
+    // Second, unlike `AutomatedTestWidgetsFlutterBinding`, which overrides
+    // `scheduleWarmUpFrame` to execute the handlers synchronously,
+    // `LiveTestWidgetsFlutterBinding` still calls them asynchronously. This
+    // allows `runApp`, which also schedules a warm-up frame, to bind a widget
+    // without rendering a frame, which is needed to for `deferFirstFrame` to
+    // take effect.
+
+    // Before `testWidgets` executes the test body, it pumps a frame with a
+    // fixed dummy widget, then calls `resetFirstFrameSent`. The pumped frame
+    // schedules a reportTiming call that has yet to arrive.
+    //
+    // This puts the test in an inconsistent state: a reportTiming callback is
+    // supposed to happen only after a frame is rendered, but due to
+    // `resetFirstFrameSent`, the framework thinks no frames have been rendered.
+
+    expect(tester.binding.sendFramesToEngine, true);
+    // Push the widget with `runApp` instead of `tester.pump`, avoiding
+    // rendering a frame, which is needed for `deferFirstFrame` later to work.
+    runApp(const DummyWidget());
+    // Verify that no widget tree is built and nothing is rendered.
+    expect(find.text('First frame'), findsNothing);
+    // Defer the first frame, making `sendFramesToEngine` false, so that widget
+    // tree will be built but not sent to the engine.
+    tester.binding.deferFirstFrame();
+    expect(tester.binding.sendFramesToEngine, false);
+    // Pump a frame, letting the reportTiming callback to run. If the
+    // reportTiming callback were to assume that `sendFramesToEngine` is true,
+    // the callback would crash.
+    await tester.pump(const Duration(milliseconds: 1));
+    await tester.binding.waitUntilFirstFrameRasterized;
+    expect(find.text('First frame'), findsOne);
+  }, skip: kIsWeb); // [intended] Web doesn't use LiveTestWidgetsFlutterBinding
+}
+
+class DummyWidget extends StatelessWidget {
+  const DummyWidget({super.key});
+
+  @override
+  Widget build(BuildContext context) {
+    return const Directionality(
+      textDirection: TextDirection.ltr,
+      child: Center(
+        child: Text('First frame'),
+      ),
+    );
+  }
+}