Remove temporary LayoutBuilder migration flag, defer markNeedsLayout (#149637)

Looks like I did not need this flag since flutter_portal pinned flutter version to 3.10 in their presubmits. 

I'll create a g3 fix for this since there're only a handful of scuba failures. Also a g3 test revealed a problem with calling `renderObject.markNeedsLayout` directly. It has to be deferred so the render tree is clean during the idle phase and the postFrameCallback phase.

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

@@ -4,6 +4,7 @@
 
 import 'package:flutter/foundation.dart';
 import 'package:flutter/rendering.dart';
+import 'package:flutter/scheduler.dart';
 
 import 'debug.dart';
 import 'framework.dart';
@@ -81,9 +82,40 @@ class _LayoutBuilderElement<ConstraintType extends Constraints> extends RenderOb
   Element? _child;
 
   @override
-  BuildScope get buildScope => LayoutBuilder.applyDoubleRebuildFix ? _buildScope : super.buildScope;
+  BuildScope get buildScope => _buildScope;
 
-  late final BuildScope _buildScope = BuildScope(scheduleRebuild: renderObject.markNeedsLayout);
+  late final BuildScope _buildScope = BuildScope(scheduleRebuild: _scheduleRebuild);
+
+  // To schedule a rebuild, markNeedsLayout needs to be called on this Element's
+  // render object (as the rebuilding is done in its performLayout call). However,
+  // the render tree should typically be kept clean during the postFrameCallbacks
+  // and the idle phase, so the layout data can be safely read.
+  bool _deferredCallbackScheduled = false;
+  void _scheduleRebuild() {
+    if (_deferredCallbackScheduled) {
+      return;
+    }
+
+    final bool deferMarkNeedsLayout = switch (SchedulerBinding.instance.schedulerPhase) {
+      SchedulerPhase.idle || SchedulerPhase.postFrameCallbacks => true,
+      SchedulerPhase.transientCallbacks || SchedulerPhase.midFrameMicrotasks || SchedulerPhase.persistentCallbacks => false,
+    };
+    if (!deferMarkNeedsLayout) {
+      renderObject.markNeedsLayout();
+      return;
+    }
+    _deferredCallbackScheduled = true;
+    SchedulerBinding.instance.scheduleFrameCallback(_frameCallback);
+  }
+
+  void _frameCallback(Duration timestamp) {
+    _deferredCallbackScheduled = false;
+    // This method is only called when the render tree is stable, if the Element
+    // is deactivated it will never be reincorporated back to the tree.
+    if (mounted) {
+      renderObject.markNeedsLayout();
+    }
+  }
 
   @override
   void visitChildren(ElementVisitor visitor) {
@@ -284,12 +316,6 @@ class LayoutBuilder extends ConstrainedLayoutBuilder<BoxConstraints> {
     required super.builder,
   });
 
-  /// Temporary flag that controls whether [LayoutBuilder]s and
-  /// [SliverLayoutBuilder]s should apply the double rebuild fix. This flag is
-  /// for migration only and **SHOULD NOT BE USED**.
-  @Deprecated('This is a temporary migration flag. DO NOT USE THIS.') // flutter_ignore: deprecation_syntax (see analyze.dart)
-  static bool applyDoubleRebuildFix = false;
-
   @override
   RenderObject createRenderObject(BuildContext context) => _RenderLayoutBuilder();
 }

packages/flutter/test/widgets/framework_test.dart

@@ -23,9 +23,6 @@ class _MyGlobalObjectKey<T extends State<StatefulWidget>> extends GlobalObjectKe
 }
 
 void main() {
-  setUp(() { LayoutBuilder.applyDoubleRebuildFix = true; });
-  tearDown(() { LayoutBuilder.applyDoubleRebuildFix = false; });
-
   testWidgets('UniqueKey control test', (WidgetTester tester) async {
     final Key key = UniqueKey();
     expect(key, hasOneLineDescription);

packages/flutter/test/widgets/layout_builder_and_global_keys_test.dart

@@ -41,9 +41,6 @@ class StatefulWrapperState extends State<StatefulWrapper> {
 }
 
 void main() {
-  setUp(() { LayoutBuilder.applyDoubleRebuildFix = true; });
-  tearDown(() { LayoutBuilder.applyDoubleRebuildFix = false; });
-
   testWidgets('Moving global key inside a LayoutBuilder', (WidgetTester tester) async {
     final GlobalKey<StatefulWrapperState> key = GlobalKey<StatefulWrapperState>();
     await tester.pumpWidget(

packages/flutter/test/widgets/layout_builder_test.dart

@@ -7,9 +7,6 @@ import 'package:flutter/widgets.dart';
 import 'package:flutter_test/flutter_test.dart';
 
 void main() {
-  setUp(() { LayoutBuilder.applyDoubleRebuildFix = true; });
-  tearDown(() { LayoutBuilder.applyDoubleRebuildFix = false; });
-
   testWidgets('LayoutBuilder parent size', (WidgetTester tester) async {
     late Size layoutBuilderSize;
     final Key childKey = UniqueKey();
@@ -334,6 +331,30 @@ void main() {
     expect(built, 2);
   });
 
+  testWidgets('LayoutBuilder does not dirty the render tree during the idle phase', (WidgetTester tester) async {
+    RenderObject? dirtyRenderObject;
+    void visitSubtree(RenderObject node) {
+      assert(dirtyRenderObject == null);
+      if (node.debugNeedsLayout) {
+        dirtyRenderObject = node;
+        return;
+      }
+      node.visitChildren(visitSubtree);
+    }
+
+    final Widget target = LayoutBuilder(
+      builder: (BuildContext context, BoxConstraints constraints) => const Placeholder(),
+    );
+    await tester.pumpWidget(target);
+    final RenderObject renderObject = tester.renderObject(find.byWidget(target));
+    visitSubtree(renderObject);
+    expect(dirtyRenderObject, isNull);
+
+    tester.element(find.byType(Placeholder)).markNeedsBuild();
+    visitSubtree(renderObject);
+    expect(dirtyRenderObject, isNull);
+  });
+
   testWidgets('LayoutBuilder can change size without rebuild', (WidgetTester tester) async {
     int built = 0;
     final Widget target = LayoutBuilder(