Add `enabled` property to `ExpansionTile` (#139519)

Adds an `enabled` property to `ExpansionTile` that allows the user to disable the internal `ListTile`, so that we can prevent user interaction.

Fixes #135770.

packages/flutter/lib/src/material/expansion_tile.dart

@@ -252,6 +252,7 @@ class ExpansionTile extends StatefulWidget {
     this.dense,
     this.visualDensity,
     this.enableFeedback = true,
+    this.enabled = true,
     this.expansionAnimationStyle,
   }) : assert(
        expandedCrossAxisAlignment != CrossAxisAlignment.baseline,
@@ -507,6 +508,15 @@ class ExpansionTile extends StatefulWidget {
   /// {@macro flutter.material.ListTile.enableFeedback}
   final bool? enableFeedback;
 
+  /// Whether this expansion tile is interactive.
+  ///
+  /// If false, the internal [ListTile] will be disabled, changing its
+  /// appearance according to the theme and disabling user interaction.
+  ///
+  /// Even if disabled, the expansion can still be toggled programmatically
+  /// through an [ExpansionTileController].
+  final bool enabled;
+
   /// Used to override the expansion animation curve and duration.
   ///
   /// If [AnimationStyle.duration] is provided, it will be used to override
@@ -685,6 +695,7 @@ class _ExpansionTileState extends State<ExpansionTile> with SingleTickerProvider
               iconColor: _iconColor.value ?? expansionTileTheme.iconColor,
               textColor: _headerColor.value,
               child: ListTile(
+                enabled: widget.enabled,
                 onTap: _handleTap,
                 dense: widget.dense,
                 visualDensity: widget.visualDensity,

packages/flutter/test/material/expansion_tile_test.dart

@@ -1333,4 +1333,36 @@ void main() {
     expect(tileWidget.enableFeedback, enableFeedback);
     expect(tileWidget.visualDensity, visualDensity);
   });
+
+  testWidgetsWithLeakTracking('ExpansionTileController should not toggle if disabled', (WidgetTester tester) async {
+    final ExpansionTileController controller = ExpansionTileController();
+
+    await tester.pumpWidget(MaterialApp(
+      home: Material(
+        child: ExpansionTile(
+          enabled: false,
+          controller: controller,
+          title: const Text('Title'),
+          children: const <Widget>[
+            Text('Child 0'),
+          ],
+        ),
+      ),
+    ));
+
+    expect(find.text('Child 0'), findsNothing);
+    expect(controller.isExpanded, isFalse);
+    await tester.tap(find.widgetWithText(ExpansionTile, 'Title'));
+    await tester.pumpAndSettle();
+    expect(find.text('Child 0'), findsNothing);
+    expect(controller.isExpanded, isFalse);
+    controller.expand();
+    await tester.pumpAndSettle();
+    expect(find.text('Child 0'), findsOneWidget);
+    expect(controller.isExpanded, isTrue);
+    await tester.tap(find.widgetWithText(ExpansionTile, 'Title'));
+    await tester.pumpAndSettle();
+    expect(find.text('Child 0'), findsOneWidget);
+    expect(controller.isExpanded, isTrue);
+  });
 }