[web] Stop using flutterViewEmbedder for platform views

lib/web_ui/lib/src/engine/dom.dart

@@ -529,6 +529,7 @@ extension DomElementExtension on DomElement {
       createDomListWrapper<DomElement>(_children);
 
   external DomElement? get firstElementChild;
+  external DomElement? get lastElementChild;
 
   external DomElement? get nextElementSibling;
 

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

@@ -432,8 +432,6 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
     };
   }
 
-  PlatformViewMessageHandler? _platformViewMessageHandler;
-
   void _sendPlatformMessage(
     String name,
     ByteData? data,
@@ -595,13 +593,7 @@ class EnginePlatformDispatcher extends ui.PlatformDispatcher {
         return;
 
       case 'flutter/platform_views':
-        _platformViewMessageHandler ??= PlatformViewMessageHandler(
-          contentManager: PlatformViewManager.instance,
-          contentHandler: (DomElement content) {
-            flutterViewEmbedder.glassPaneElement.append(content);
-          },
-        );
-        _platformViewMessageHandler!.handlePlatformViewCall(data, callback!);
+        implicitView!.platformViewMessageHandler.handlePlatformViewCall(data, callback!);
         return;
 
       case 'flutter/accessibility':

lib/web_ui/lib/src/engine/platform_views/message_handler.dart

@@ -5,9 +5,11 @@
 import 'dart:typed_data';
 
 import '../dom.dart';
+import '../platform_dispatcher.dart';
 import '../services.dart';
 import '../util.dart';
 import 'content_manager.dart';
+import 'slots.dart';
 
 /// The signature for a callback for a Platform Message. From the `ui` package.
 /// Copied here so there's no circular dependencies.
@@ -40,24 +42,23 @@ typedef PlatformViewContentHandler = void Function(DomElement);
 /// [HtmlViewEmbedder.disposeViews]
 class PlatformViewMessageHandler {
   PlatformViewMessageHandler({
-    required PlatformViewManager contentManager,
-    PlatformViewContentHandler? contentHandler,
-  }) : _contentManager = contentManager,
-       _contentHandler = contentHandler;
+    required DomElement platformViewsContainer,
+    PlatformViewManager? contentManager,
+  }) : _contentManager = contentManager ?? PlatformViewManager.instance,
+       _platformViewsContainer = platformViewsContainer;
 
   final MethodCodec _codec = const StandardMethodCodec();
   final PlatformViewManager _contentManager;
-  final PlatformViewContentHandler? _contentHandler;
+  final DomElement _platformViewsContainer;
 
   /// Handle a `create` Platform View message.
   ///
   /// This will attempt to render the `contents` and of a Platform View, if its
   /// `viewType` has been registered previously.
   ///
-  /// (See [PlatformViewContentManager.registerFactory] for more details.)
+  /// (See [PlatformViewManager.registerFactory] for more details.)
   ///
-  /// The `contents` are delegated to a [_contentHandler] function, so the
-  /// active rendering backend can inject them in the right place of the DOM.
+  /// The `contents` are inserted into the [_platformViewsContainer].
   ///
   /// If all goes well, this function will `callback` with an empty success envelope.
   /// In case of error, this will `callback` with an error envelope describing the error.
@@ -98,7 +99,7 @@ class PlatformViewMessageHandler {
 
     // For now, we don't need anything fancier. If needed, this can be converted
     // to a PlatformViewStrategy class for each web-renderer backend?
-    _contentHandler?.call(content);
+    _platformViewsContainer.append(content);
     callback(_codec.encodeSuccessEnvelope(null));
   }
 

lib/web_ui/lib/src/engine/window.dart

@@ -21,6 +21,7 @@ import 'mouse/context_menu.dart';
 import 'mouse/cursor.dart';
 import 'navigation/history.dart';
 import 'platform_dispatcher.dart';
+import 'platform_views/message_handler.dart';
 import 'services.dart';
 import 'util.dart';
 
@@ -39,6 +40,7 @@ const int kImplicitViewId = 0;
 abstract interface class EngineFlutterView extends ui.FlutterView {
   ContextMenu get contextMenu;
   MouseCursor get mouseCursor;
+  PlatformViewMessageHandler get platformViewMessageHandler;
   DomElement get rootElement;
 }
 
@@ -75,6 +77,10 @@ class EngineFlutterWindow extends ui.SingletonFlutterWindow implements EngineFlu
   @override
   DomElement get rootElement => flutterViewEmbedder.flutterViewElement;
 
+  @override
+  late final PlatformViewMessageHandler platformViewMessageHandler =
+      PlatformViewMessageHandler(platformViewsContainer: flutterViewEmbedder.glassPaneElement);
+
   /// Handles the browser history integration to allow users to use the back
   /// button, etc.
   BrowserHistory get browserHistory {

lib/web_ui/test/engine/platform_views/message_handler_test.dart

@@ -24,18 +24,17 @@ void testMain() {
       const int viewId = 6;
       late PlatformViewManager contentManager;
       late Completer<ByteData?> completer;
-      late Completer<DomElement> contentCompleter;
 
       setUp(() {
         contentManager = PlatformViewManager();
         completer = Completer<ByteData?>();
-        contentCompleter = Completer<DomElement>();
       });
 
       group('"create" message', () {
         test('unregistered viewType, fails with descriptive exception',
             () async {
           final PlatformViewMessageHandler messageHandler = PlatformViewMessageHandler(
+            platformViewsContainer: createDomElement('div'),
             contentManager: contentManager,
           );
           final ByteData? message = _getCreateMessage(viewType, viewId);
@@ -57,6 +56,7 @@ void testMain() {
               viewType, (int id) => createDomHTMLDivElement());
           contentManager.renderContent(viewType, viewId, null);
           final PlatformViewMessageHandler messageHandler = PlatformViewMessageHandler(
+            platformViewsContainer: createDomElement('div'),
             contentManager: contentManager,
           );
           final ByteData? message = _getCreateMessage(viewType, viewId);
@@ -77,6 +77,7 @@ void testMain() {
           contentManager.registerFactory(
               viewType, (int id) => createDomHTMLDivElement()..id = 'success');
           final PlatformViewMessageHandler messageHandler = PlatformViewMessageHandler(
+            platformViewsContainer: createDomElement('div'),
             contentManager: contentManager,
           );
           final ByteData? message = _getCreateMessage(viewType, viewId);
@@ -89,27 +90,37 @@ void testMain() {
                   'The response should be a success envelope, with null in it.');
         });
 
-        test('calls a contentHandler with the result of creating a view',
+        test('inserts the created view into the platformViewsContainer',
             () async {
+          final DomElement platformViewsContainer = createDomElement('pv-container');
           contentManager.registerFactory(
               viewType, (int id) => createDomHTMLDivElement()..id = 'success');
           final PlatformViewMessageHandler messageHandler = PlatformViewMessageHandler(
+            platformViewsContainer: platformViewsContainer,
             contentManager: contentManager,
-            contentHandler: contentCompleter.complete,
           );
           final ByteData? message = _getCreateMessage(viewType, viewId);
 
           messageHandler.handlePlatformViewCall(message, completer.complete);
 
-          final DomElement contents = await contentCompleter.future;
           final ByteData? response = await completer.future;
 
-          expect(contents.querySelector('div#success'), isNotNull,
-              reason:
-                  'The element created by the factory should be present in the created view.');
-          expect(codec.decodeEnvelope(response!), isNull,
-              reason:
-                  'The response should be a success envelope, with null in it.');
+          expect(
+            platformViewsContainer.children.single,
+            isNotNull,
+            reason: 'The container has a single child, the created view.',
+          );
+          final DomElement platformView = platformViewsContainer.children.single;
+          expect(
+            platformView.querySelector('div#success'),
+            isNotNull,
+            reason: 'The element created by the factory should be present in the created view.',
+          );
+          expect(
+            codec.decodeEnvelope(response!),
+            isNull,
+            reason: 'The response should be a success envelope, with null in it.',
+          );
         });
 
         test('passes creation params to the factory', () async {
@@ -119,6 +130,7 @@ void testMain() {
             return createDomHTMLDivElement();
           });
           final PlatformViewMessageHandler messageHandler = PlatformViewMessageHandler(
+            platformViewsContainer: createDomElement('div'),
             contentManager: contentManager,
           );
 
@@ -177,8 +189,10 @@ void testMain() {
             return Object();
           });
 
-          final PlatformViewMessageHandler messageHandler =
-              PlatformViewMessageHandler(contentManager: contentManager);
+          final PlatformViewMessageHandler messageHandler = PlatformViewMessageHandler(
+            platformViewsContainer: createDomElement('div'),
+            contentManager: contentManager,
+          );
           final ByteData? message = _getCreateMessage(viewType, viewId);
 
           expect(() {
@@ -196,6 +210,7 @@ void testMain() {
 
         test('never fails, even for unknown viewIds', () async {
           final PlatformViewMessageHandler messageHandler = PlatformViewMessageHandler(
+            platformViewsContainer: createDomElement('div'),
             contentManager: contentManager,
           );
           final ByteData? message = _getDisposeMessage(viewId);
@@ -210,6 +225,7 @@ void testMain() {
 
         test('never fails, even for unknown viewIds', () async {
           final PlatformViewMessageHandler messageHandler = PlatformViewMessageHandler(
+            platformViewsContainer: createDomElement('div'),
             contentManager: _FakePlatformViewManager(viewIdCompleter.complete),
           );
           final ByteData? message = _getDisposeMessage(viewId);