[canvaskit] Fix bug when overlays are disabled

lib/web_ui/lib/src/engine/canvaskit/embedded_views.dart

@@ -38,7 +38,13 @@ class HtmlViewEmbedder {
   /// This causes all drawing to go to a single canvas, with all of the platform
   /// views rendered over top. This may result in incorrect rendering with
   /// platform views.
-  static const bool disableOverlays = maximumSurfaces <= 1;
+  static bool get disableOverlays =>
+      debugDisableOverlays || maximumSurfaces <= 1;
+
+  /// Force the view embedder to disable overlays.
+  ///
+  /// This should never be used outside of tests.
+  static bool debugDisableOverlays = false;
 
   /// The set of platform views using the backup surface.
   final Set<int> _viewsUsingBackupSurface = <int>{};
@@ -170,12 +176,14 @@ class HtmlViewEmbedder {
   CkCanvas? compositeEmbeddedView(int viewId) {
     final int compositedViewCount = _compositionOrder.length;
     _compositionOrder.add(viewId);
-    if (compositedViewCount < _pictureRecordersCreatedDuringPreroll.length) {
-      _pictureRecorders[viewId] =
-          _pictureRecordersCreatedDuringPreroll[compositedViewCount];
-    } else {
-      _viewsUsingBackupSurface.add(viewId);
-      _pictureRecorders[viewId] = _backupPictureRecorder!;
+    if (!disableOverlays) {
+      if (compositedViewCount < _pictureRecordersCreatedDuringPreroll.length) {
+        _pictureRecorders[viewId] =
+            _pictureRecordersCreatedDuringPreroll[compositedViewCount];
+      } else {
+        _viewsUsingBackupSurface.add(viewId);
+        _pictureRecorders[viewId] = _backupPictureRecorder!;
+      }
     }
 
     // Do nothing if this view doesn't need to be composited.

lib/web_ui/test/canvaskit/embedded_views_test.dart

@@ -574,10 +574,34 @@ void testMain() {
       // The below line should not throw an error.
       dispatcher.rasterizer!.draw(sb.build().layerTree);
       expect(
-          domRenderer.glassPaneElement!
+          domRenderer.glassPaneShadow!
               .querySelectorAll('flt-platform-view-slot'),
           isEmpty);
     });
+
+    test('does not crash when overlays are disabled', () async {
+      HtmlViewEmbedder.debugDisableOverlays = true;
+      ui.platformViewRegistry.registerViewFactory(
+        'test-platform-view',
+        (int viewId) => html.DivElement()..id = 'view-0',
+      );
+      await _createPlatformView(0, 'test-platform-view');
+
+      final EnginePlatformDispatcher dispatcher =
+          ui.window.platformDispatcher as EnginePlatformDispatcher;
+
+      final LayerSceneBuilder sb = LayerSceneBuilder();
+      sb.pushOffset(0, 0);
+      sb.addPlatformView(0, width: 10, height: 10);
+      sb.pop();
+      // The below line should not throw an error.
+      dispatcher.rasterizer!.draw(sb.build().layerTree);
+      expect(
+          domRenderer.glassPaneShadow!
+              .querySelectorAll('flt-platform-view-slot'),
+          hasLength(1));
+      HtmlViewEmbedder.debugDisableOverlays = false;
+    });
     // TODO(dit): https://github.com/flutter/flutter/issues/60040
   }, skip: isIosSafari);
 }