In the semantics tree, do not detach a child if it has already been assigned a new parent (#6773)

Fixes https://github.com/flutter/flutter/issues/6690

Also add a version of the Gallery smoke test that enables semantics

examples/flutter_gallery/test/smoke_test.dart

@@ -5,9 +5,10 @@
 import 'dart:collection' show LinkedHashSet;
 
 import 'package:flutter/material.dart';
+import 'package:flutter/rendering.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:flutter_gallery/gallery/item.dart' show GalleryItem, kAllGalleryItems;
-import 'package:flutter_gallery/main.dart' as flutter_gallery_main;
+import 'package:flutter_gallery/gallery/app.dart' show GalleryApp;
 
 const String kCaption = 'Flutter Gallery';
 
@@ -47,48 +48,55 @@ Future<Null> smokeDemo(WidgetTester tester, String routeName) async {
   return null;
 }
 
+Future<Null> runSmokeTest(WidgetTester tester) async {
+  await tester.pumpWidget(new GalleryApp());
+  await tester.pump(); // see https://github.com/flutter/flutter/issues/1865
+  await tester.pump(); // triggers a frame
+
+  expect(find.text(kCaption), findsOneWidget);
+
+  final List<double> scrollDeltas = new List<double>();
+  double previousY = tester.getTopRight(find.text(demoCategories[0])).y;
+  for (String routeName in routeNames) {
+    final double y = tester.getTopRight(findGalleryItemByRouteName(tester, routeName)).y;
+    scrollDeltas.add(previousY - y);
+    previousY = y;
+  }
+
+  // Launch each demo and then scroll that item out of the way.
+  for (int i = 0; i < routeNames.length; i += 1) {
+    final String routeName = routeNames[i];
+    await smokeDemo(tester, routeName);
+    await tester.scroll(findGalleryItemByRouteName(tester, routeName), new Offset(0.0, scrollDeltas[i]));
+    await tester.pump(); // start the scroll
+    await tester.pump(const Duration(milliseconds: 500)); // wait for overscroll to timeout, if necessary
+    await tester.pump(const Duration(seconds: 3)); // wait for overscroll to fade away, if necessary
+    tester.binding.debugAssertNoTransientCallbacks('A transient callback was still active after leaving route $routeName');
+  }
+
+  Finder navigationMenuButton = find.byTooltip('Open navigation menu');
+  expect(navigationMenuButton, findsOneWidget);
+  await tester.tap(navigationMenuButton);
+  await tester.pump(); // Start opening drawer.
+  await tester.pump(const Duration(seconds: 1)); // Wait until it's really opened.
+
+  // switch theme
+  await tester.tap(find.text('Dark'));
+  await tester.pump();
+  await tester.pump(const Duration(seconds: 1)); // Wait until it's changed.
+
+  // switch theme
+  await tester.tap(find.text('Light'));
+  await tester.pump();
+  await tester.pump(const Duration(seconds: 1)); // Wait until it's changed.
+}
+
 void main() {
+  testWidgets('Flutter Gallery app smoke test', runSmokeTest);
+
   testWidgets('Flutter Gallery app smoke test', (WidgetTester tester) async {
-    flutter_gallery_main
+    RendererBinding.instance.setSemanticsEnabled(true);
-        .main(); // builds the app and schedules a frame but doesn't trigger one
+    await runSmokeTest(tester);
-    await tester.pump(); // see https://github.com/flutter/flutter/issues/1865
+    RendererBinding.instance.setSemanticsEnabled(false);
-    await tester.pump(); // triggers a frame
-
-    expect(find.text(kCaption), findsOneWidget);
-
-    final List<double> scrollDeltas = new List<double>();
-    double previousY = tester.getTopRight(find.text(demoCategories[0])).y;
-    for (String routeName in routeNames) {
-      final double y = tester.getTopRight(findGalleryItemByRouteName(tester, routeName)).y;
-      scrollDeltas.add(previousY - y);
-      previousY = y;
-    }
-
-    // Launch each demo and then scroll that item out of the way.
-    for (int i = 0; i < routeNames.length; i += 1) {
-      final String routeName = routeNames[i];
-      await smokeDemo(tester, routeName);
-      await tester.scroll(findGalleryItemByRouteName(tester, routeName), new Offset(0.0, scrollDeltas[i]));
-      await tester.pump(); // start the scroll
-      await tester.pump(const Duration(milliseconds: 500)); // wait for overscroll to timeout, if necessary
-      await tester.pump(const Duration(seconds: 3)); // wait for overscroll to fade away, if necessary
-      tester.binding.debugAssertNoTransientCallbacks('A transient callback was still active after leaving route $routeName');
-    }
-
-    Finder navigationMenuButton = find.byTooltip('Open navigation menu');
-    expect(navigationMenuButton, findsOneWidget);
-    await tester.tap(navigationMenuButton);
-    await tester.pump(); // Start opening drawer.
-    await tester.pump(const Duration(seconds: 1)); // Wait until it's really opened.
-
-    // switch theme
-    await tester.tap(find.text('Dark'));
-    await tester.pump();
-    await tester.pump(const Duration(seconds: 1)); // Wait until it's changed.
-
-    // switch theme
-    await tester.tap(find.text('Light'));
-    await tester.pump();
-    await tester.pump(const Duration(seconds: 1)); // Wait until it's changed.
   });
 }

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

@@ -138,7 +138,11 @@ abstract class RendererBinding extends BindingBase implements SchedulerBinding,
   SemanticsHandle _semanticsHandle;
 
   void _handleSemanticsEnabledChanged() {
-    if (ui.window.semanticsEnabled) {
+    setSemanticsEnabled(ui.window.semanticsEnabled);
+  }
+
+  void setSemanticsEnabled(bool enabled) {
+    if (enabled) {
       _semanticsHandle ??= _pipelineOwner.ensureSemantics();
     } else {
       _semanticsHandle?.dispose();

packages/flutter/lib/src/rendering/semantics.dart

@@ -372,6 +372,7 @@ class SemanticsNode extends AbstractNode {
   /// child list for the given frame at once instead of needing to process the
   /// changes incrementally as new children are compiled.
   void finalizeChildren() {
+    // The goal of this function is updating sawChange.
     if (_children != null) {
       for (SemanticsNode child in _children)
         child._dead = true;
@@ -473,8 +474,12 @@ class SemanticsNode extends AbstractNode {
     owner._detachedNodes.add(this);
     super.detach();
     if (_children != null) {
-      for (SemanticsNode child in _children)
+      for (SemanticsNode child in _children) {
-        child.detach();
+        // The list of children may be stale and may contain nodes that have
+        // been assigned to a different parent.
+        if (child.parent == this)
+          child.detach();
+      }
     }
   }