Fix `PopupMenuButton`'s `IconButton` not inheriting `IconTheme` size (#100199)

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

@@ -37,6 +37,7 @@ const double _kMenuMinWidth = 2.0 * _kMenuWidthStep;
 const double _kMenuVerticalPadding = 8.0;
 const double _kMenuWidthStep = 56.0;
 const double _kMenuScreenPadding = 8.0;
+const double _kDefaultIconSize = 24.0;
 
 /// Used to configure how the [PopupMenuButton] positions its popup menu.
 enum PopupMenuPosition {
@@ -1088,7 +1089,9 @@ class PopupMenuButton<T> extends StatefulWidget {
 
   /// If provided, the size of the [Icon].
   ///
-  /// If this property is null, the default size is 24.0 pixels.
+  /// If this property is null, then [IconThemeData.size] is used.
+  /// If [IconThemeData.size] is also null, then
+  /// default size is 24.0 pixels.
   final double? iconSize;
 
   /// Optional size constraints for the menu.
@@ -1190,6 +1193,7 @@ class PopupMenuButtonState<T> extends State<PopupMenuButton<T>> {
 
   @override
   Widget build(BuildContext context) {
+    final IconThemeData iconTheme = IconTheme.of(context);
     final bool enableFeedback = widget.enableFeedback
       ?? PopupMenuTheme.of(context).enableFeedback
       ?? true;
@@ -1212,7 +1216,7 @@ class PopupMenuButtonState<T> extends State<PopupMenuButton<T>> {
       icon: widget.icon ?? Icon(Icons.adaptive.more),
       padding: widget.padding,
       splashRadius: widget.splashRadius,
-      iconSize: widget.iconSize ?? 24.0,
+      iconSize: widget.iconSize ?? iconTheme.size ?? _kDefaultIconSize,
       tooltip: widget.tooltip ?? MaterialLocalizations.of(context).showMenuTooltip,
       onPressed: widget.enabled ? showButtonMenu : null,
       enableFeedback: enableFeedback,

packages/flutter/test/material/popup_menu_test.dart

@@ -2698,6 +2698,51 @@ void main() {
 
     expect(tester.getTopLeft(find.byWidgetPredicate((Widget w) => '${w.runtimeType}' == '_PopupMenu<int?>')), const Offset(8.0, 90.0));
   });
+
+  testWidgets("PopupMenuButton icon inherits IconTheme's size", (WidgetTester tester) async {
+    Widget _buildPopupMenu({double? themeIconSize, double? iconSize}) {
+      return MaterialApp(
+        theme: ThemeData(
+          iconTheme: IconThemeData(
+            size: themeIconSize,
+          ),
+        ),
+        home: Scaffold(
+          body: Center(
+            child: PopupMenuButton<String>(
+              iconSize: iconSize,
+              itemBuilder: (_) => <PopupMenuEntry<String>>[
+                const PopupMenuItem<String>(
+                  value: 'value',
+                  child: Text('Item 0'),
+                ),
+              ],
+            ),
+          ),
+        ),
+      );
+    }
+
+    // Popup menu with default icon size.
+    await tester.pumpWidget(_buildPopupMenu());
+    IconButton iconButton = tester.widget(find.widgetWithIcon(IconButton, Icons.more_vert));
+    // Default PopupMenuButton icon size is 24.0.
+    expect(iconButton.iconSize, 24.0);
+
+    // Popup menu with custom theme icon size.
+    await tester.pumpWidget(_buildPopupMenu(themeIconSize: 30.0));
+    await tester.pumpAndSettle();
+    iconButton = tester.widget(find.widgetWithIcon(IconButton, Icons.more_vert));
+    // PopupMenuButton icon inherits IconTheme's size.
+    expect(iconButton.iconSize, 30.0);
+
+    // Popup menu with custom icon size.
+    await tester.pumpWidget(_buildPopupMenu(themeIconSize: 30.0, iconSize: 50.0));
+    await tester.pumpAndSettle();
+    iconButton = tester.widget(find.widgetWithIcon(IconButton, Icons.more_vert));
+    // PopupMenuButton icon size overrides IconTheme's size.
+    expect(iconButton.iconSize, 50.0);
+  });
 }
 
 class TestApp extends StatefulWidget {