Wires up expanded state in web engine (#164048)

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fixes https://github.com/flutter/flutter/issues/162141

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- [ ] I listed at least one issue that this PR fixes in the description
above.
- [ ] I updated/added relevant documentation (doc comments with `///`).
- [ ] I added new tests to check the change I am making, or this PR is
[test-exempt].
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---------

Co-authored-by: Mouad Debbar <mouad.debbar@gmail.com>

engine/src/flutter/ci/licenses_golden/licenses_flutter

@@ -42685,6 +42685,7 @@ ORIGIN: ../../../flutter/lib/web_ui/lib/src/engine/scene_view.dart + ../../../fl
 ORIGIN: ../../../flutter/lib/web_ui/lib/src/engine/semantics.dart + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/lib/src/engine/semantics/accessibility.dart + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/lib/src/engine/semantics/checkable.dart + ../../../flutter/LICENSE
+ORIGIN: ../../../flutter/lib/web_ui/lib/src/engine/semantics/expandable.dart + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/lib/src/engine/semantics/focusable.dart + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/lib/src/engine/semantics/header.dart + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/lib/web_ui/lib/src/engine/semantics/heading.dart + ../../../flutter/LICENSE
@@ -45617,6 +45618,7 @@ FILE: ../../../flutter/lib/web_ui/lib/src/engine/scene_view.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/semantics.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/semantics/accessibility.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/semantics/checkable.dart
+FILE: ../../../flutter/lib/web_ui/lib/src/engine/semantics/expandable.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/semantics/focusable.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/semantics/header.dart
 FILE: ../../../flutter/lib/web_ui/lib/src/engine/semantics/heading.dart

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

@@ -105,6 +105,7 @@ export 'engine/scene_painting.dart';
 export 'engine/scene_view.dart';
 export 'engine/semantics/accessibility.dart';
 export 'engine/semantics/checkable.dart';
+export 'engine/semantics/expandable.dart';
 export 'engine/semantics/focusable.dart';
 export 'engine/semantics/header.dart';
 export 'engine/semantics/heading.dart';

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

@@ -4,6 +4,7 @@
 
 export 'semantics/accessibility.dart';
 export 'semantics/checkable.dart';
+export 'semantics/expandable.dart';
 export 'semantics/focusable.dart';
 export 'semantics/header.dart';
 export 'semantics/heading.dart';

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

@@ -0,0 +1,27 @@
+// 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 'semantics.dart';
+
+/// Adds expandability behavior to a semantic node.
+///
+/// An expandable node would have the `aria-expanded` attribute set to "true" if the node
+/// is currently expanded (i.e. [SemanticsObject.isExpanded] is true), and set
+/// to "false" if it's not expanded (i.e. [SemanticsObject.isExpanded] is
+/// false). If the node is not expandable (i.e. [SemanticsObject.isExpandable]
+/// is false), then `aria-expanded` is unset.
+class Expandable extends SemanticBehavior {
+  Expandable(super.semanticsObject, super.owner);
+
+  @override
+  void update() {
+    if (semanticsObject.isFlagsDirty) {
+      if (semanticsObject.isExpandable) {
+        owner.setAttribute('aria-expanded', semanticsObject.isExpanded);
+      } else {
+        owner.removeAttribute('aria-expanded');
+      }
+    }
+  }
+}

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

@@ -20,6 +20,7 @@ import '../vector_math.dart';
 import '../window.dart';
 import 'accessibility.dart';
 import 'checkable.dart';
+import 'expandable.dart';
 import 'focusable.dart';
 import 'header.dart';
 import 'heading.dart';
@@ -461,6 +462,7 @@ abstract class SemanticRole {
     addRouteName();
     addLabelAndValue(preferredRepresentation: preferredLabelRepresentation);
     addSelectableBehavior();
+    addExpandableBehavior();
   }
 
   /// Initializes a blank role for a [semanticsObject].
@@ -627,6 +629,10 @@ abstract class SemanticRole {
     }
   }
 
+  void addExpandableBehavior() {
+    addSemanticBehavior(Expandable(semanticsObject, this));
+  }
+
   /// Adds a semantic behavior to this role.
   ///
   /// This method should be called by concrete implementations of
@@ -1947,6 +1953,19 @@ class SemanticsObject {
   /// selected.
   bool get isSelected => hasFlag(ui.SemanticsFlag.isSelected);
 
+  /// If true, this node represents something that can be annotated as
+  /// "expanded", such as a expansion tile or drop down menu
+  ///
+  /// Expandability is managed by `aria-expanded`.
+  ///
+  /// See also:
+  ///
+  ///   * [isExpanded], which indicates whether the node is currently selected.
+  bool get isExpandable => hasFlag(ui.SemanticsFlag.hasExpandedState);
+
+  /// Indicates whether the node is currently expanded.
+  bool get isExpanded => hasFlag(ui.SemanticsFlag.isExpanded);
+
   /// Role-specific adjustment of the vertical position of the child container.
   ///
   /// This is used, for example, by the [SemanticScrollable] to compensate for the

engine/src/flutter/lib/web_ui/test/engine/semantics/semantics_test.dart

@@ -86,6 +86,9 @@ void runSemanticsTests() {
   group('selectables', () {
     _testSelectables();
   });
+  group('expandables', () {
+    _testExpandables();
+  });
   group('tappable', () {
     _testTappable();
   });
@@ -2328,6 +2331,77 @@ void _testSelectables() {
   });
 }
 
+void _testExpandables() {
+  test('renders and updates non-expandable, expanded, and unexpanded nodes', () async {
+    semantics()
+      ..debugOverrideTimestampFunction(() => _testTime)
+      ..semanticsEnabled = true;
+
+    final tester = SemanticsTester(owner());
+    tester.updateNode(
+      id: 0,
+      rect: const ui.Rect.fromLTRB(0, 0, 100, 60),
+      children: <SemanticsNodeUpdate>[
+        tester.updateNode(id: 1, isSelectable: false, rect: const ui.Rect.fromLTRB(0, 0, 100, 20)),
+        tester.updateNode(
+          id: 2,
+          isExpandable: true,
+          isExpanded: false,
+          rect: const ui.Rect.fromLTRB(0, 20, 100, 40),
+        ),
+        tester.updateNode(
+          id: 3,
+          isExpandable: true,
+          isExpanded: true,
+          rect: const ui.Rect.fromLTRB(0, 40, 100, 60),
+        ),
+      ],
+    );
+    tester.apply();
+
+    expectSemanticsTree(owner(), '''
+<sem>
+  <sem-c>
+    <sem></sem>
+    <sem aria-expanded="false"></sem>
+    <sem aria-expanded="true"></sem>
+  </sem-c>
+</sem>
+''');
+
+    // Missing attributes cannot be expressed using HTML patterns, so check directly.
+    final nonExpandable = owner().debugSemanticsTree![1]!.element;
+    expect(nonExpandable.getAttribute('aria-expanded'), isNull);
+
+    // Flip the values and check that that ARIA attribute is updated.
+    tester.updateNode(
+      id: 2,
+      isExpandable: true,
+      isExpanded: true,
+      rect: const ui.Rect.fromLTRB(0, 20, 100, 40),
+    );
+    tester.updateNode(
+      id: 3,
+      isExpandable: true,
+      isExpanded: false,
+      rect: const ui.Rect.fromLTRB(0, 40, 100, 60),
+    );
+    tester.apply();
+
+    expectSemanticsTree(owner(), '''
+<sem>
+  <sem-c>
+    <sem></sem>
+    <sem aria-expanded="true"></sem>
+    <sem aria-expanded="false"></sem>
+  </sem-c>
+</sem>
+''');
+
+    semantics().semanticsEnabled = false;
+  });
+}
+
 void _testTappable() {
   test('renders an enabled button', () async {
     semantics()

engine/src/flutter/lib/web_ui/test/engine/semantics/semantics_tester.dart

@@ -34,6 +34,8 @@ class SemanticsTester {
     bool? isChecked,
     bool? isSelectable,
     bool? isSelected,
+    bool? isExpandable,
+    bool? isExpanded,
     bool? isButton,
     bool? isLink,
     bool? isTextField,
@@ -130,6 +132,12 @@ class SemanticsTester {
     if (isSelected ?? false) {
       flags |= ui.SemanticsFlag.isSelected.index;
     }
+    if (isExpandable ?? false) {
+      flags |= ui.SemanticsFlag.hasExpandedState.index;
+    }
+    if (isExpanded ?? false) {
+      flags |= ui.SemanticsFlag.isExpanded.index;
+    }
     if (isButton ?? false) {
       flags |= ui.SemanticsFlag.isButton.index;
     }