[web] Fix Scene clip bounds. Trigger resize on DPR Change. (flutter/engine#50161)

The Scene of the HTML renderer is passing incorrect size information to the engine, and when DPR<1, it can result in elements being culled off of the viewport.

In addition to that, when an app is embedded, not all changes in DPR cause a Resize event (only those some of the dimensions fails by a rounding error!), so this PR ensures that all DPR events in embedded trigger a resize event.

### Issues

Fixes https://github.com/flutter/flutter/issues/129182

### Testing

Looking good at: https://dit-astral-test.web.app

[C++, Objective-C, Java style guides]: https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style

engine/src/flutter/ci/licenses_golden/licenses_flutter

@@ -6134,6 +6134,7 @@ ORIGIN: ../../../flutter/lib/web_ui/lib/src/engine/vector_math.dart + ../../../f
 ORIGIN: ../../../flutter/lib/web_ui/lib/src/engine/view_embedder/dimensions_provider/custom_element_dimensions_provider.dart + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/lib/src/engine/view_embedder/dimensions_provider/dimensions_provider.dart + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/lib/src/engine/view_embedder/dimensions_provider/full_page_dimensions_provider.dart + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/lib/web_ui/lib/src/engine/view_embedder/display_dpr_stream.dart + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/lib/src/engine/view_embedder/dom_manager.dart + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/lib/src/engine/view_embedder/embedding_strategy/custom_element_embedding_strategy.dart + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/lib/src/engine/view_embedder/embedding_strategy/embedding_strategy.dart + ../../../flutter/LICENSE
@@ -8997,6 +8998,7 @@ FILE: ../../../flutter/lib/web_ui/lib/src/engine/vector_math.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/view_embedder/dimensions_provider/custom_element_dimensions_provider.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/view_embedder/dimensions_provider/dimensions_provider.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/view_embedder/dimensions_provider/full_page_dimensions_provider.dart
+FILE: ../../../flutter/lib/web_ui/lib/src/engine/view_embedder/display_dpr_stream.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/view_embedder/dom_manager.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/view_embedder/embedding_strategy/custom_element_embedding_strategy.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/view_embedder/embedding_strategy/embedding_strategy.dart

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

@@ -190,6 +190,7 @@ export 'engine/vector_math.dart';
 export 'engine/view_embedder/dimensions_provider/custom_element_dimensions_provider.dart';
 export 'engine/view_embedder/dimensions_provider/dimensions_provider.dart';
 export 'engine/view_embedder/dimensions_provider/full_page_dimensions_provider.dart';
+export 'engine/view_embedder/display_dpr_stream.dart';
 export 'engine/view_embedder/dom_manager.dart';
 export 'engine/view_embedder/embedding_strategy/custom_element_embedding_strategy.dart';
 export 'engine/view_embedder/embedding_strategy/embedding_strategy.dart';

engine/src/flutter/lib/web_ui/lib/src/engine/html/scene.dart

@@ -45,9 +45,15 @@ class PersistedScene extends PersistedContainerSurface {
 
   @override
   void recomputeTransformAndClip() {
-    // The scene clip is the size of the entire window.
-    final ui.Size screen = window.physicalSize;
-    localClipBounds = ui.Rect.fromLTRB(0, 0, screen.width, screen.height);
+    // The scene clip is the size of the entire window **in Logical pixels**.
+    //
+    // Even though the majority of the engine uses `physicalSize`, there are some
+    // bits (like the HTML renderer, or dynamic view sizing) that are implemented
+    // using CSS, and CSS operates in logical pixels.
+    //
+    // See also: [EngineFlutterView.resize].
+    final ui.Size bounds = window.physicalSize / window.devicePixelRatio;
+    localClipBounds = ui.Rect.fromLTRB(0, 0, bounds.width, bounds.height);
     projectedClip = null;
   }
 

engine/src/flutter/lib/web_ui/lib/src/engine/view_embedder/dimensions_provider/custom_element_dimensions_provider.dart

@@ -4,6 +4,7 @@
 
 import 'dart:async';
 
+import 'package:ui/src/engine/display.dart';
 import 'package:ui/src/engine/dom.dart';
 import 'package:ui/src/engine/window.dart';
 import 'package:ui/ui.dart' as ui show Size;
@@ -12,21 +13,36 @@ import 'dimensions_provider.dart';
 
 /// This class provides observable, real-time dimensions of a host element.
 ///
+/// This class needs a `Stream` of `devicePixelRatio` changes, like the one
+/// provided by [DisplayDprStream], because html resize observers do not report
+/// DPR changes.
+///
 /// All the measurements returned from this class are potentially *expensive*,
 /// and should be cached as needed. Every call to every method on this class
 /// WILL perform actual DOM measurements.
+///
+/// This broadcasts `null` size events, to match the implementation of the
+/// FullPageDimensionsProvider, but it could broadcast the size coming from the
+/// DomResizeObserverEntry. Further changes in the engine are required for this
+/// to be effective.
 class CustomElementDimensionsProvider extends DimensionsProvider {
   /// Creates a [CustomElementDimensionsProvider] from a [_hostElement].
-  CustomElementDimensionsProvider(this._hostElement) {
+  CustomElementDimensionsProvider(this._hostElement, {
+    Stream<double>? onDprChange,
+  }) {
+    // Send a resize event when the page DPR changes.
+    _dprChangeStreamSubscription = onDprChange?.listen((_) {
+      _broadcastSize(null);
+    });
+
     // Hook up a resize observer on the hostElement (if supported!).
     _hostElementResizeObserver = createDomResizeObserver((
       List<DomResizeObserverEntry> entries,
       DomResizeObserver _,
     ) {
-      entries
-          .map((DomResizeObserverEntry entry) =>
-              ui.Size(entry.contentRect.width, entry.contentRect.height))
-          .forEach(_broadcastSize);
+      for (final DomResizeObserverEntry _ in entries) {
+        _broadcastSize(null);
+      }
     });
 
     assert(() {
@@ -45,11 +61,12 @@ class CustomElementDimensionsProvider extends DimensionsProvider {
 
   // Handle resize events
   late DomResizeObserver? _hostElementResizeObserver;
-  final StreamController<ui.Size> _onResizeStreamController =
-      StreamController<ui.Size>.broadcast();
+  late StreamSubscription<double>? _dprChangeStreamSubscription;
+  final StreamController<ui.Size?> _onResizeStreamController =
+      StreamController<ui.Size?>.broadcast();
 
   // Broadcasts the last seen `Size`.
-  void _broadcastSize(ui.Size size) {
+  void _broadcastSize(ui.Size? size) {
     _onResizeStreamController.add(size);
   }
 
@@ -58,16 +75,17 @@ class CustomElementDimensionsProvider extends DimensionsProvider {
     super.close();
     _hostElementResizeObserver?.disconnect();
     // ignore:unawaited_futures
+    _dprChangeStreamSubscription?.cancel();
+    // ignore:unawaited_futures
     _onResizeStreamController.close();
   }
 
   @override
-  Stream<ui.Size> get onResize => _onResizeStreamController.stream;
+  Stream<ui.Size?> get onResize => _onResizeStreamController.stream;
 
   @override
   ui.Size computePhysicalSize() {
-    final double devicePixelRatio = getDevicePixelRatio();
-
+    final double devicePixelRatio = EngineFlutterDisplay.instance.devicePixelRatio;
     return ui.Size(
       _hostElement.clientWidth * devicePixelRatio,
       _hostElement.clientHeight * devicePixelRatio,

engine/src/flutter/lib/web_ui/lib/src/engine/view_embedder/dimensions_provider/dimensions_provider.dart

@@ -5,11 +5,11 @@
 import 'dart:async';
 
 import 'package:meta/meta.dart';
+import 'package:ui/src/engine/dom.dart';
+import 'package:ui/src/engine/view_embedder/display_dpr_stream.dart';
 import 'package:ui/src/engine/window.dart';
 import 'package:ui/ui.dart' as ui show Size;
 
-import '../../display.dart';
-import '../../dom.dart';
 import 'custom_element_dimensions_provider.dart';
 import 'full_page_dimensions_provider.dart';
 
@@ -32,18 +32,15 @@ abstract class DimensionsProvider {
   /// Creates the appropriate DimensionsProvider depending on the incoming [hostElement].
   factory DimensionsProvider.create({DomElement? hostElement}) {
     if (hostElement != null) {
-      return CustomElementDimensionsProvider(hostElement);
+      return CustomElementDimensionsProvider(
+        hostElement,
+        onDprChange: DisplayDprStream.instance.dprChanged,
+      );
     } else {
       return FullPageDimensionsProvider();
     }
   }
 
-  /// Returns the DPI reported by the browser.
-  double getDevicePixelRatio() {
-    // This is overridable in tests.
-    return EngineFlutterDisplay.instance.devicePixelRatio;
-  }
-
   /// Returns the [ui.Size] of the "viewport".
   ///
   /// This function is expensive. It triggers browser layout if there are
@@ -57,6 +54,16 @@ abstract class DimensionsProvider {
   );
 
   /// Returns a Stream with the changes to [ui.Size] (when cheap to get).
+  ///
+  /// Currently this Stream always returns `null` measurements because the
+  /// resize event that we use for [FullPageDimensionsProvider] does not contain
+  /// the new size, so users of this Stream everywhere immediately retrieve the
+  /// new `physicalSize` from the window.
+  ///
+  /// The [CustomElementDimensionsProvider] *could* broadcast the new size, but
+  /// to keep both implementations consistent (and their consumers), for now all
+  /// events from this Stream are going to be `null` (until we find a performant
+  /// way to retrieve the dimensions in full-page mode).
   Stream<ui.Size?> get onResize;
 
   /// Whether the [DimensionsProvider] instance has been closed or not.

engine/src/flutter/lib/web_ui/lib/src/engine/view_embedder/dimensions_provider/full_page_dimensions_provider.dart

@@ -5,6 +5,7 @@
 import 'dart:async';
 
 import 'package:ui/src/engine/browser_detection.dart';
+import 'package:ui/src/engine/display.dart';
 import 'package:ui/src/engine/dom.dart';
 import 'package:ui/src/engine/window.dart';
 import 'package:ui/ui.dart' as ui show Size;
@@ -67,7 +68,7 @@ class FullPageDimensionsProvider extends DimensionsProvider {
     late double windowInnerWidth;
     late double windowInnerHeight;
     final DomVisualViewport? viewport = domWindow.visualViewport;
-    final double devicePixelRatio = getDevicePixelRatio();
+    final double devicePixelRatio = EngineFlutterDisplay.instance.devicePixelRatio;
 
     if (viewport != null) {
       if (operatingSystem == OperatingSystem.iOs) {
@@ -102,7 +103,7 @@ class FullPageDimensionsProvider extends DimensionsProvider {
     double physicalHeight,
     bool isEditingOnMobile,
   ) {
-    final double devicePixelRatio = getDevicePixelRatio();
+    final double devicePixelRatio = EngineFlutterDisplay.instance.devicePixelRatio;
     final DomVisualViewport? viewport = domWindow.visualViewport;
     late double windowInnerHeight;
 

engine/src/flutter/lib/web_ui/lib/src/engine/view_embedder/display_dpr_stream.dart

@@ -0,0 +1,93 @@
+// 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:async';
+import 'dart:js_interop';
+
+import 'package:meta/meta.dart';
+import 'package:ui/src/engine/display.dart';
+import 'package:ui/src/engine/dom.dart';
+import 'package:ui/ui.dart' as ui show Display;
+
+/// Provides a stream of `devicePixelRatio` changes for the given display.
+///
+/// Note that until the Window Management API is generally available, this class
+/// only monitors the global `devicePixelRatio` property, provided by the default
+/// [EngineFlutterDisplay.instance].
+///
+/// See: https://developer.mozilla.org/en-US/docs/Web/API/Window_Management_API
+class DisplayDprStream {
+  DisplayDprStream(
+    this._display, {
+    @visibleForTesting DebugDisplayDprStreamOverrides? overrides,
+  })  : _currentDpr = _display.devicePixelRatio,
+        _debugOverrides = overrides {
+    // Start listening to DPR changes.
+    _subscribeToMediaQuery();
+  }
+
+  /// A singleton instance of DisplayDprStream.
+  static DisplayDprStream instance =
+      DisplayDprStream(EngineFlutterDisplay.instance);
+
+  // The display object that will provide the DPR information.
+  final ui.Display _display;
+
+  // Last reported value of DPR.
+  double _currentDpr;
+
+  // Controls the [dprChanged] broadcast Stream.
+  final StreamController<double> _dprStreamController =
+      StreamController<double>.broadcast();
+
+  // Object that fires a `change` event for the `_currentDpr`.
+  late DomEventTarget _dprMediaQuery;
+
+  // Creates the media query for the latest known DPR value, and adds a change listener to it.
+  void _subscribeToMediaQuery() {
+    if (_debugOverrides?.getMediaQuery != null) {
+      _dprMediaQuery = _debugOverrides!.getMediaQuery!(_currentDpr);
+    } else {
+      _dprMediaQuery = domWindow.matchMedia('(resolution: ${_currentDpr}dppx)');
+    }
+    _dprMediaQuery.addEventListenerWithOptions(
+      'change',
+      createDomEventListener(_onDprMediaQueryChange),
+      <String, Object>{
+        // We only listen `once` because this event only triggers once when the
+        // DPR changes from `_currentDpr`. Once that happens, we need a new
+        // `_dprMediaQuery` that is watching the new `_currentDpr`.
+        //
+        // By using `once`, we don't need to worry about detaching the event
+        // listener from the old mediaQuery after we're done with it.
+        'once': true,
+        'passive': true,
+      },
+    );
+  }
+
+  // Handler of the _dprMediaQuery 'change' event.
+  //
+  // This calls subscribe again because events are listened to with `once: true`.
+  JSVoid _onDprMediaQueryChange(DomEvent _) {
+    _currentDpr = _display.devicePixelRatio;
+    _dprStreamController.add(_currentDpr);
+    // Re-subscribe...
+    _subscribeToMediaQuery();
+  }
+
+  /// A stream that emits the latest value of [EngineFlutterDisplay.instance.devicePixelRatio].
+  Stream<double> get dprChanged => _dprStreamController.stream;
+
+  // The overrides object that is used for testing.
+  final DebugDisplayDprStreamOverrides? _debugOverrides;
+}
+
+@visibleForTesting
+class DebugDisplayDprStreamOverrides {
+  DebugDisplayDprStreamOverrides({
+    this.getMediaQuery,
+  });
+  final DomEventTarget Function(double currentValue)? getMediaQuery;
+}

engine/src/flutter/lib/web_ui/test/engine/view_embedder/dimensions_provider/custom_element_dimensions_provider_test.dart

@@ -2,9 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-@TestOn('browser')
-library;
-
 import 'dart:async';
 
 import 'package:test/bootstrap/browser.dart';
@@ -109,23 +106,48 @@ void doTests() {
     });
 
     test('funnels resize events on sizeSource', () async {
+      EngineFlutterDisplay.instance.debugOverrideDevicePixelRatio(2.7);
+
       sizeSource
         ..style.width = '100px'
         ..style.height = '100px';
 
-      expect(await provider.onResize.first, const ui.Size(100, 100));
+      expect(provider.onResize.first, completes);
+      expect(provider.computePhysicalSize(), const ui.Size(270, 270));
 
       sizeSource
         ..style.width = '200px'
         ..style.height = '200px';
 
-      expect(await provider.onResize.first, const ui.Size(200, 200));
+      expect(provider.onResize.first, completes);
+      expect(provider.computePhysicalSize(), const ui.Size(540, 540));
 
       sizeSource
         ..style.width = '300px'
         ..style.height = '300px';
 
-      expect(await provider.onResize.first, const ui.Size(300, 300));
+      expect(provider.onResize.first, completes);
+      expect(provider.computePhysicalSize(), const ui.Size(810, 810));
+    });
+
+    test('funnels DPR change events too', () async {
+      // Override the source of DPR events...
+      final StreamController<double> dprController =
+          StreamController<double>.broadcast();
+
+      // Inject the dprController stream into the CustomElementDimensionsProvider.
+      final CustomElementDimensionsProvider provider =
+          CustomElementDimensionsProvider(
+        sizeSource,
+        onDprChange: dprController.stream,
+      );
+
+      // Set and broadcast the mock DPR value
+      EngineFlutterDisplay.instance.debugOverrideDevicePixelRatio(3.2);
+      dprController.add(3.2);
+
+      expect(provider.onResize.first, completes);
+      expect(provider.computePhysicalSize(), const ui.Size(32, 32));
     });
 
     test('closed by onHotRestart', () async {

engine/src/flutter/lib/web_ui/test/engine/view_embedder/dimensions_provider/dimensions_provider_test.dart

@@ -2,9 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-@TestOn('browser')
-library;
-
 import 'package:test/bootstrap/browser.dart';
 import 'package:test/test.dart';
 import 'package:ui/src/engine.dart';
@@ -31,15 +28,4 @@ void doTests() {
       expect(provider, isA<CustomElementDimensionsProvider>());
     });
   });
-
-  group('getDevicePixelRatio', () {
-    test('Returns the correct pixelRatio', () async {
-      // Override the DPI to something known, but weird...
-      EngineFlutterDisplay.instance.debugOverrideDevicePixelRatio(33930);
-
-      final DimensionsProvider provider = DimensionsProvider.create();
-
-      expect(provider.getDevicePixelRatio(), 33930);
-    });
-  });
 }

engine/src/flutter/lib/web_ui/test/engine/view_embedder/dimensions_provider/full_page_dimensions_provider_test.dart

@@ -2,9 +2,6 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-@TestOn('browser')
-library;
-
 import 'dart:async';
 
 import 'package:test/bootstrap/browser.dart';

engine/src/flutter/lib/web_ui/test/engine/view_embedder/display_dpr_stream_test.dart

@@ -0,0 +1,45 @@
+// 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:async';
+
+import 'package:test/bootstrap/browser.dart';
+import 'package:test/test.dart';
+import 'package:ui/src/engine.dart';
+
+void main() {
+  internalBootstrapBrowserTest(() => doTests);
+}
+
+void doTests() {
+  final DomEventTarget eventTarget = createDomElement('div');
+
+  group('dprChanged Stream', () {
+    late DisplayDprStream dprStream;
+
+    setUp(() async {
+      dprStream = DisplayDprStream(EngineFlutterDisplay.instance,
+          overrides: DebugDisplayDprStreamOverrides(
+            getMediaQuery: (_) => eventTarget,
+          ));
+    });
+
+    test('funnels display DPR on every mediaQuery "change" event.', () async {
+      final Future<List<double>> dprs = dprStream.dprChanged
+          .take(3)
+          .timeout(const Duration(seconds: 1))
+          .toList();
+
+      // Simulate the events
+      EngineFlutterDisplay.instance.debugOverrideDevicePixelRatio(6.9);
+      eventTarget.dispatchEvent(createDomEvent('Event', 'change'));
+      EngineFlutterDisplay.instance.debugOverrideDevicePixelRatio(4.2);
+      eventTarget.dispatchEvent(createDomEvent('Event', 'change'));
+      EngineFlutterDisplay.instance.debugOverrideDevicePixelRatio(0.71);
+      eventTarget.dispatchEvent(createDomEvent('Event', 'change'));
+
+      expect(await dprs, <double>[6.9, 4.2, 0.71]);
+    });
+  });
+}