fix widget built twice during warm up frame (#39079)

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

@@ -794,6 +794,17 @@ mixin WidgetsBinding on BindingBase, SchedulerBinding, GestureBinding, RendererB
   Element get renderViewElement => _renderViewElement;
   Element _renderViewElement;
 
+  /// Schedules a [Timer] for attaching the root widget.
+  ///
+  /// This is called by [runApp] to configure the widget tree. Consider using
+  /// [attachRootWidget] if you want to build the widget tree synchronously.
+  @protected
+  void scheduleAttachRootWidget(Widget rootWidget) {
+    Timer.run(() {
+      attachRootWidget(rootWidget);
+    });
+  }
+
   /// Takes a widget and attaches it to the [renderViewElement], creating it if
   /// necessary.
   ///
@@ -855,7 +866,7 @@ mixin WidgetsBinding on BindingBase, SchedulerBinding, GestureBinding, RendererB
 ///    ensure the widget, element, and render trees are all built.
 void runApp(Widget app) {
   WidgetsFlutterBinding.ensureInitialized()
-    ..attachRootWidget(app)
+    ..scheduleAttachRootWidget(app)
     ..scheduleWarmUpFrame();
 }
 

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

@@ -498,13 +498,8 @@ class _NestedScrollCoordinator implements ScrollActivityDelegate, ScrollHoldCont
 
   bool get hasScrolledBody {
     for (_NestedScrollPosition position in _innerPositions) {
-      // TODO(chunhtai): Replace null check with assert once
-      // https://github.com/flutter/flutter/issues/31195 is fixed.
-      if (
-        position.minScrollExtent != null &&
-        position.pixels != null &&
-        position.pixels > position.minScrollExtent
-      ) {
+      assert(position.minScrollExtent != null && position.pixels != null);
+      if (position.pixels > position.minScrollExtent) {
         return true;
       }
     }

packages/flutter/test/widgets/nested_scroll_view_async_test.dart

-// Copyright 2019 The Chromium 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/material.dart';
-import 'package:flutter_test/flutter_test.dart';
-import 'package:flutter/gestures.dart' show DragStartBehavior;
-import 'package:quiver/testing/async.dart';
-
-void main() {
-  setUp(() {
-    WidgetsFlutterBinding.ensureInitialized();
-    WidgetsBinding.instance.resetEpoch();
-  });
-
-  test('NestedScrollView can build sccessfully if mark dirty during warm up frame', () {
-    final FakeAsync fakeAsync = FakeAsync();
-    fakeAsync.run((FakeAsync async) {
-      runApp(
-        MaterialApp(
-          home: Material(
-            child: DefaultTabController(
-              length: 1,
-              child: NestedScrollView(
-                dragStartBehavior: DragStartBehavior.down,
-                headerSliverBuilder: (BuildContext context, bool innerBoxIsScrolled) {
-                  return <Widget>[
-                    const SliverPersistentHeader(
-                      delegate: TestHeader(),
-                    ),
-                  ];
-                },
-                body: SingleChildScrollView(
-                  dragStartBehavior: DragStartBehavior.down,
-                  child: Container(
-                    height: 1000.0,
-                    child: const Placeholder(),
-                  ),
-                ),
-              ),
-            ),
-          ),
-        ),
-      );
-      // Marks element as dirty right before the first draw frame is called.
-      // This can happen when engine flush user setting.
-      final Element element = find.byType(NestedScrollView, skipOffstage: false).evaluate().single;
-      element.markNeedsBuild();
-      // Triggers draw frame timer scheduled in scheduleWarmUpFrame.
-      fakeAsync.flushTimers();
-    });
-    // Make sure widget is rebuilt correctly.
-    expect(
-      find.byType(NestedScrollView, skipOffstage: false).evaluate().single.widget is NestedScrollView,
-      isTrue
-    );
-  });
-}
-
-class TestHeader extends SliverPersistentHeaderDelegate {
-  const TestHeader();
-  @override
-  double get minExtent => 100.0;
-  @override
-  double get maxExtent => 100.0;
-  @override
-  Widget build(BuildContext context, double shrinkOffset, bool overlapsContent) {
-    return const Placeholder();
-  }
-  @override
-  bool shouldRebuild(TestHeader oldDelegate) => false;
-}

packages/flutter/test/widgets/run_app_async_test.dart

+// Copyright 2016 The Chromium 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/material.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:quiver/testing/async.dart';
+
+void main() {
+  setUp(() {
+    WidgetsFlutterBinding.ensureInitialized();
+    WidgetsBinding.instance.resetEpoch();
+  });
+
+  test('WidgetBinding build rendering tree and warm up frame back to back', () {
+    final FakeAsync fakeAsync = FakeAsync();
+    fakeAsync.run((FakeAsync async) {
+      runApp(
+        const MaterialApp(
+          home: Material(
+            child: Text('test'),
+          ),
+        ),
+      );
+      // Rendering tree is not built synchronously.
+      expect(WidgetsBinding.instance.renderViewElement, isNull);
+      fakeAsync.flushTimers();
+      expect(WidgetsBinding.instance.renderViewElement, isNotNull);
+    });
+  });
+}

packages/flutter_test/lib/src/binding.dart

@@ -993,6 +993,14 @@ class AutomatedTestWidgetsFlutterBinding extends TestWidgetsFlutterBinding {
     _currentFakeAsync.flushMicrotasks();
   }
 
+  @override
+  void scheduleAttachRootWidget(Widget rootWidget) {
+    // We override the default version of this so that the application-startup widget tree
+    // build does not schedule timers which we might never get around to running.
+    attachRootWidget(rootWidget);
+    _currentFakeAsync.flushMicrotasks();
+  }
+
   @override
   Future<void> idle() {
     final Future<void> result = super.idle();