Fix bug in multisurfacerenderer where canvases do not have "position: absolute" (#181053)

When using MultiSurfaceRasterizer, the on-screen canvases did not have
`position: absolute`, causing layout issues when platform views with
overlaid content was rendered.

Fixes internal bug b/475770800

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engine/src/flutter/lib/web_ui/lib/src/engine/compositing/canvas_provider.dart

@@ -105,6 +105,7 @@ class OnscreenCanvasProvider extends CanvasProvider<DomHTMLCanvasElement> {
     final cssHeight = '${size.height / ratio}px';
     canvas.style
       ..width = cssWidth
-      ..height = cssHeight;
+      ..height = cssHeight
+      ..position = 'absolute';
   }
 }

engine/src/flutter/lib/web_ui/lib/src/engine/renderer.dart

@@ -67,6 +67,10 @@ abstract class Renderer {
 
   @mustCallSuper
   FutureOr<void> initialize() {
+    _setUpViewListeners();
+  }
+
+  void _setUpViewListeners() {
     // Views may have been registered before this renderer was initialized.
     // Create rasterizers for them and then start listening for new view
     // creation/disposal events.
@@ -347,8 +351,12 @@ abstract class Renderer {
   /// Clears the state of this renderer. Used in tests.
   @mustCallSuper
   void debugClear() {
+    _onViewCreatedListener.cancel();
+    _onViewDisposedListener.cancel();
     for (final ViewRasterizer rasterizer in rasterizers.values) {
-      rasterizer.debugClear();
+      rasterizer.dispose();
     }
+    rasterizers.clear();
+    _setUpViewListeners();
   }
 }

engine/src/flutter/lib/web_ui/test/ui/platform_view_position_test.dart

@@ -0,0 +1,74 @@
+// Copyright 2013 The Flutter 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 'dart:js_interop';
+
+import 'package:test/bootstrap/browser.dart';
+import 'package:test/test.dart';
+import 'package:ui/src/engine.dart' as engine;
+import 'package:ui/src/engine.dart';
+import 'package:ui/ui.dart' as ui;
+import 'package:ui/ui_web/src/ui_web.dart' as ui_web;
+
+import '../common/rendering.dart';
+import '../common/test_initialization.dart';
+import 'utils.dart';
+
+void main() {
+  internalBootstrapBrowserTest(() => testMain);
+}
+
+Future<void> testMain() async {
+  setUpUnitTests(withImplicitView: true, setUpTestViewDimensions: false);
+
+  test('Onscreen canvas has position: absolute', () async {
+    // Force multi-surface mode to ensure OnscreenCanvasProvider is used.
+    engine.debugOverrideJsConfiguration(
+      <String, Object?>{'canvasKitForceMultiSurfaceRasterizer': true}.jsify()
+          as engine.JsFlutterConfiguration?,
+    );
+    // Reset the renderer to ensure it is created with the new configuration.
+    engine.renderer.debugResetRasterizer();
+    engine.renderer.debugClear();
+
+    const platformViewType = 'test-platform-view';
+    ui_web.platformViewRegistry.registerViewFactory(platformViewType, (int viewId) {
+      final DomElement element = createDomHTMLDivElement();
+      element.id = 'view-$viewId';
+      return element;
+    });
+
+    await createPlatformView(0, platformViewType);
+
+    // Create a scene with a platform view and some drawing to trigger canvas creation.
+    final recorder = ui.PictureRecorder();
+    final canvas = ui.Canvas(recorder);
+    canvas.drawRect(
+      const ui.Rect.fromLTWH(0, 0, 100, 100),
+      ui.Paint()..color = const ui.Color(0xFFFF0000),
+    );
+    final ui.Picture picture = recorder.endRecording();
+
+    final sb = ui.SceneBuilder();
+    sb.pushOffset(0, 0);
+    sb.addPlatformView(0, width: 100, height: 100);
+    sb.addPicture(ui.Offset.zero, picture);
+
+    await renderScene(sb.build());
+
+    // Find the canvas element.
+    final DomElement canvasElement = (implicitView as engine.EngineFlutterView).dom.sceneHost
+        .querySelectorAll('canvas')
+        .single;
+
+    // Verify position is absolute.
+    expect(
+      canvasElement.style.position,
+      'absolute',
+      reason: 'Canvas should have position: absolute',
+    );
+
+    engine.debugOverrideJsConfiguration(null);
+  });
+}