Move the accesibility placeholder to the platform dispatcher.

lib/web_ui/lib/src/engine/platform_dispatcher.dart

@@ -7,6 +7,7 @@ import 'dart:convert';
 import 'dart:js_interop';
 import 'dart:typed_data';
 
+import 'package:meta/meta.dart';
 import 'package:ui/ui.dart' as ui;
 import 'package:ui/ui_web/src/ui_web.dart' as ui_web;
 
@@ -79,6 +80,7 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
     registerHotRestartListener(dispose);
     AppLifecycleState.instance.addListener(_setAppLifecycleState);
     ViewFocusBinding.instance.addListener(invokeOnViewFocusChange);
+    domDocument.body?.append(accessibilityPlaceholder);
     _onViewDisposedListener = viewManager.onViewDisposed.listen((_) {
       // Send a metrics changed event to the framework when a view is disposed.
       // View creation/resize is handled by the `_didResize` handler in the
@@ -93,6 +95,12 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
   static EnginePlatformDispatcher get instance => _instance;
   static final EnginePlatformDispatcher _instance = EnginePlatformDispatcher();
 
+  @visibleForTesting
+  final DomElement accessibilityPlaceholder = EngineSemantics
+    .instance
+    .semanticsHelper
+    .prepareAccessibilityPlaceholder();
+
   PlatformConfiguration configuration = PlatformConfiguration(
     locales: parseBrowserLanguages(),
     textScaleFactor: findBrowserTextScaleFactor(),
@@ -116,6 +124,7 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
     HighContrastSupport.instance.removeListener(_updateHighContrast);
     AppLifecycleState.instance.removeListener(_setAppLifecycleState);
     ViewFocusBinding.instance.removeListener(invokeOnViewFocusChange);
+    accessibilityPlaceholder.remove();
     _onViewDisposedListener.cancel();
     viewManager.dispose();
   }

lib/web_ui/lib/src/engine/view_embedder/dom_manager.dart

@@ -8,7 +8,6 @@ import '../configuration.dart';
 import '../dom.dart';
 import '../platform_views/content_manager.dart';
 import '../safe_browser_api.dart';
-import '../semantics/semantics.dart';
 import 'style_manager.dart';
 
 /// Manages DOM elements and the DOM structure for a [ui.FlutterView].
@@ -71,11 +70,6 @@ class DomManager {
 
     // Rendering host (shadow root) children.
 
-    final DomElement accessibilityPlaceholder = EngineSemantics
-        .instance.semanticsHelper
-        .prepareAccessibilityPlaceholder();
-
-    renderingHost.append(accessibilityPlaceholder);
     renderingHost.append(sceneHost);
     renderingHost.append(announcementsHost);
 

lib/web_ui/test/engine/platform_dispatcher/platform_dispatcher_test.dart

@@ -22,6 +22,16 @@ void testMain() {
   });
 
   group('PlatformDispatcher', () {
+    late EnginePlatformDispatcher dispatcher;
+
+    setUp(() {
+      dispatcher = EnginePlatformDispatcher();
+    });
+
+    tearDown(() {
+      dispatcher.dispose();
+    });
+
     test('reports at least one display', () {
       expect(ui.PlatformDispatcher.instance.displays.length, greaterThan(0));
     });
@@ -30,15 +40,16 @@ void testMain() {
       final MockHighContrastSupport mockHighContrast =
           MockHighContrastSupport();
       HighContrastSupport.instance = mockHighContrast;
-      final EnginePlatformDispatcher engineDispatcher =
+
+      final EnginePlatformDispatcher dispatcher =
           EnginePlatformDispatcher();
 
-      expect(engineDispatcher.accessibilityFeatures.highContrast, isTrue);
+      expect(dispatcher.accessibilityFeatures.highContrast, isTrue);
       mockHighContrast.isEnabled = false;
       mockHighContrast.invokeListeners(mockHighContrast.isEnabled);
-      expect(engineDispatcher.accessibilityFeatures.highContrast, isFalse);
+      expect(dispatcher.accessibilityFeatures.highContrast, isFalse);
 
-      engineDispatcher.dispose();
+      dispatcher.dispose();
     });
 
     test('AppLifecycleState transitions through all states', () {
@@ -295,7 +306,6 @@ void testMain() {
     });
 
     test('disposes all its views', () {
-      final EnginePlatformDispatcher dispatcher = EnginePlatformDispatcher();
       final EngineFlutterView view1 =
           EngineFlutterView(dispatcher, createDomHTMLDivElement());
       final EngineFlutterView view2 =
@@ -319,7 +329,6 @@ void testMain() {
     });
 
     test('connects view disposal to metrics changed event', () {
-      final EnginePlatformDispatcher dispatcher = EnginePlatformDispatcher();
       final EngineFlutterView view1 =
           EngineFlutterView(dispatcher, createDomHTMLDivElement());
       final EngineFlutterView view2 =
@@ -347,7 +356,6 @@ void testMain() {
     });
 
     test('disconnects view disposal event on dispose', () {
-      final EnginePlatformDispatcher dispatcher = EnginePlatformDispatcher();
       final EngineFlutterView view1 =
           EngineFlutterView(dispatcher, createDomHTMLDivElement());
 
@@ -367,7 +375,6 @@ void testMain() {
     });
 
     test('invokeOnViewFocusChange calls onViewFocusChange', () {
-      final EnginePlatformDispatcher dispatcher = EnginePlatformDispatcher();
       final List<ui.ViewFocusEvent> dispatchedViewFocusEvents = <ui.ViewFocusEvent>[];
       const ui.ViewFocusEvent viewFocusEvent = ui.ViewFocusEvent(
         viewId: 0,
@@ -383,7 +390,6 @@ void testMain() {
     });
 
     test('invokeOnViewFocusChange preserves the zone', () {
-      final EnginePlatformDispatcher dispatcher = EnginePlatformDispatcher();
       final Zone zone1 = Zone.current.fork();
       final Zone zone2 = Zone.current.fork();
       const ui.ViewFocusEvent viewFocusEvent = ui.ViewFocusEvent(
@@ -402,6 +408,15 @@ void testMain() {
         dispatcher.invokeOnViewFocusChange(viewFocusEvent);
       });
     });
+
+    test('appends an accesibility placeholder', () {
+      expect(dispatcher.accessibilityPlaceholder.isConnected, isTrue);
+    });
+
+    test('removes the accesibility placeholder', () {
+      dispatcher.dispose();
+      expect(dispatcher.accessibilityPlaceholder.isConnected, isFalse);
+    });
   });
 }
 

lib/web_ui/test/engine/semantics/semantics_auto_enable_test.dart

@@ -31,11 +31,7 @@ Future<void> testMain() async {
             .instance.accessibilityFeatures.accessibleNavigation,
         isFalse);
 
-    final DomShadowRoot renderingHost =
-        EnginePlatformDispatcher.instance.implicitView!.dom.renderingHost;
-
-    final DomElement placeholder =
-        renderingHost.querySelector('flt-semantics-placeholder')!;
+    final DomElement placeholder = domDocument.querySelector('flt-semantics-placeholder')!;
 
     expect(placeholder.isConnected, isTrue);
 

lib/web_ui/test/engine/semantics/semantics_placeholder_enable_test.dart

@@ -26,10 +26,7 @@ Future<void> testMain() async {
     expect(semantics().semanticsEnabled, isFalse);
 
     // Synthesize a click on the placeholder.
-    final DomShadowRoot renderingHost =
-        EnginePlatformDispatcher.instance.implicitView!.dom.renderingHost;
-    final DomElement placeholder =
-        renderingHost.querySelector('flt-semantics-placeholder')!;
+    final DomElement placeholder = domDocument.querySelector('flt-semantics-placeholder')!;
 
     expect(placeholder.isConnected, isTrue);
 

lib/web_ui/test/engine/view_embedder/dom_manager_test.dart

@@ -39,11 +39,10 @@ void doTests() {
       expect(rootChildren[3].tagName, equalsIgnoringCase('style'));
 
       final List<DomElement> shadowChildren = domManager.renderingHost.childNodes.cast<DomElement>().toList();
-      expect(shadowChildren.length, 4);
-      expect(shadowChildren[0].tagName, equalsIgnoringCase('flt-semantics-placeholder'));
-      expect(shadowChildren[1], domManager.sceneHost);
-      expect(shadowChildren[2], domManager.announcementsHost);
-      expect(shadowChildren[3].tagName, equalsIgnoringCase('style'));
+      expect(shadowChildren.length, 3);
+      expect(shadowChildren[0], domManager.sceneHost);
+      expect(shadowChildren[1], domManager.announcementsHost);
+      expect(shadowChildren[2].tagName, equalsIgnoringCase('style'));
     });
 
     test('hide placeholder text for textfield', () {