Add support for ExpansionPanel custom splash color (#147126)

Relates to https://github.com/flutter/flutter/pull/147098 and https://github.com/flutter/flutter/issues/147097

Aside from fixing the expland/collapse icon color in the other PR, I noticed the splash color on both the icon button and the full expansion panel (if ExpansionPanel.canTapOnHeader is set to true) is just `Theme.of(context).splashColor` on the `InkWell` and `Theme.of(context).highlightColor` on the `IconButton` which may not suit the color scheme of the `ExpansionPanel`, so I have added a custom field `splashColor`, which will effect both the `IconButton` and the full panel `Inkwell`.

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

@@ -39,6 +39,8 @@ class ExpandIcon extends StatefulWidget {
     this.color,
     this.disabledColor,
     this.expandedColor,
+    this.splashColor,
+    this.highlightColor,
   });
 
   /// Whether the icon is in an expanded state.
@@ -94,6 +96,20 @@ class ExpandIcon extends StatefulWidget {
   /// and for [dark theme](https://material.io/design/color/dark-theme.html#ui-application)
   final Color? expandedColor;
 
+  /// Defines the splash color of the IconButton.
+  ///
+  /// If [ThemeData.useMaterial3] is true, this field will be ignored,
+  /// as [IconButton.splashColor] will be ignored, and you should use
+  /// [highlightColor] instead.
+  ///
+  /// Defaults to [ThemeData.splashColor].
+  final Color? splashColor;
+
+  /// Defines the highlight color of the IconButton.
+  ///
+  /// Defaults to [ThemeData.highlightColor].
+  final Color? highlightColor;
+
   @override
   State<ExpandIcon> createState() => _ExpandIconState();
 }
@@ -170,6 +186,8 @@ class _ExpandIconState extends State<ExpandIcon> with SingleTickerProviderStateM
       child: IconButton(
         padding: widget.padding,
         iconSize: widget.size,
+        highlightColor: widget.highlightColor,
+        splashColor: widget.splashColor,
         color: _iconColor,
         disabledColor: widget.disabledColor,
         onPressed: widget.onPressed == null ? null : _handlePressed,

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

@@ -6,6 +6,7 @@ import 'package:flutter/widgets.dart';
 
 import 'constants.dart';
 import 'expand_icon.dart';
+import 'icon_button.dart';
 import 'ink_well.dart';
 import 'material_localizations.dart';
 import 'mergeable_material.dart';
@@ -80,6 +81,8 @@ class ExpansionPanel {
     this.isExpanded = false,
     this.canTapOnHeader = false,
     this.backgroundColor,
+    this.splashColor,
+    this.highlightColor,
   });
 
   /// The widget builder that builds the expansion panels' header.
@@ -95,6 +98,28 @@ class ExpansionPanel {
   /// Defaults to false.
   final bool isExpanded;
 
+  /// Defines the splash color of the panel if [canTapOnHeader] is true,
+  /// or the splash color of the expand/collapse IconButton if [canTapOnHeader]
+  /// is false.
+  ///
+  /// If [canTapOnHeader] is false, and [ThemeData.useMaterial3] is
+  /// true, this field will be ignored, as [IconButton.splashColor]
+  /// will be ignored, and you should use [highlightColor] instead.
+  ///
+  /// If this is null, then the icon button will use its default splash color
+  /// [ThemeData.splashColor], and the panel will use its default splash color
+  /// [ThemeData.splashColor] (if [canTapOnHeader] is true).
+  final Color? splashColor;
+
+  /// Defines the highlight color of the panel if [canTapOnHeader] is true, or
+  /// the highlight color of the expand/collapse IconButton if [canTapOnHeader]
+  /// is false.
+  ///
+  /// If this is null, then the icon button will use its default highlight color
+  /// [ThemeData.highlightColor], and the panel will use its default highlight
+  /// color [ThemeData.highlightColor] (if [canTapOnHeader] is true).
+  final Color? highlightColor;
+
   /// Whether tapping on the panel's header will expand/collapse it.
   ///
   /// Defaults to false.
@@ -125,6 +150,8 @@ class ExpansionPanelRadio extends ExpansionPanel {
     required super.body,
     super.canTapOnHeader,
     super.backgroundColor,
+    super.splashColor,
+    super.highlightColor,
   });
 
   /// The value that uniquely identifies a radio panel so that the currently
@@ -366,6 +393,8 @@ class _ExpansionPanelListState extends State<ExpansionPanelList> {
           disabledColor: child.canTapOnHeader ? widget.expandIconColor : null,
           isExpanded: _isChildExpanded(index),
           padding: _kExpandIconPadding,
+          splashColor: child.splashColor,
+          highlightColor: child.highlightColor,
           onPressed: !child.canTapOnHeader
               ? (bool isExpanded) => _handlePressed(isExpanded, index)
               : null,
@@ -399,6 +428,8 @@ class _ExpansionPanelListState extends State<ExpansionPanelList> {
       if (child.canTapOnHeader) {
         header = MergeSemantics(
           child: InkWell(
+            splashColor: child.splashColor,
+            highlightColor: child.highlightColor,
             onTap: () => _handlePressed(_isChildExpanded(index), index),
             child: header,
           ),

packages/flutter/test/material/expansion_panel_test.dart

@@ -1948,6 +1948,74 @@ void main() {
     }
   });
 
+  testWidgets('Ensure IconButton splashColor and highlightColor are correctly set when canTapOnHeader is false', (WidgetTester tester) async {
+    const Color expectedSplashColor = Colors.green;
+    const Color expectedHighlightColor = Colors.yellow;
+
+    await tester.pumpWidget(MaterialApp(
+      home: SingleChildScrollView(
+        child: ExpansionPanelList(
+          children: <ExpansionPanel>[
+            ExpansionPanel(
+              headerBuilder: (BuildContext context, bool isExpanded) {
+                return const ListTile(title: Text('Panel 1'));
+              },
+              body: const ListTile(title: Text('Content for Panel 1')),
+              splashColor: expectedSplashColor,
+              highlightColor: expectedHighlightColor,
+            ),
+          ],
+        ),
+      ),
+    ));
+
+    await tester.tap(find.text('Panel 1'));
+    await tester.pumpAndSettle();
+
+    final IconButton iconButton = tester.widget(find.byType(IconButton).first);
+    expect(iconButton.splashColor, expectedSplashColor);
+    expect(iconButton.highlightColor, expectedHighlightColor);
+  });
+
+  testWidgets('Ensure InkWell splashColor and highlightColor are correctly set when canTapOnHeader is true', (WidgetTester tester) async {
+    const Color expectedSplashColor = Colors.green;
+    const Color expectedHighlightColor = Colors.yellow;
+
+    await tester.pumpWidget(MaterialApp(
+      home: SingleChildScrollView(
+        child: ExpansionPanelList(
+          children: <ExpansionPanel>[
+            ExpansionPanel(
+              canTapOnHeader: true,
+              headerBuilder: (BuildContext context, bool isExpanded) {
+                return Container(
+                  padding: const EdgeInsets.all(16),
+                  alignment: Alignment.centerLeft,
+                  child: const Text('Panel 1'),
+                );
+              },
+              body: const ListTile(title: Text('Content for Panel 1')),
+              splashColor: expectedSplashColor,
+              highlightColor: expectedHighlightColor,
+            ),
+          ],
+        ),
+      ),
+    ));
+
+    await tester.pumpAndSettle();
+
+    final Finder inkWellFinder = find.descendant(
+      of: find.byType(ExpansionPanelList),
+      matching: find.byWidgetPredicate((Widget widget) =>
+        widget is InkWell && widget.onTap != null)
+    );
+
+    final InkWell inkWell = tester.widget<InkWell>(inkWellFinder);
+    expect(inkWell.splashColor, expectedSplashColor);
+    expect(inkWell.highlightColor, expectedHighlightColor);
+  });
+
   testWidgets('ExpandIcon.disabledColor uses expandIconColor color when canTapOnHeader is true', (WidgetTester tester) async {
     const Color expandIconColor = Color(0xff0000ff);