[Reland]: Fix `DropdownButton` menu clip (#104251)

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

@@ -286,27 +286,33 @@ class _DropdownMenuState<T> extends State<_DropdownMenu<T>> {
           namesRoute: true,
           explicitChildNodes: true,
           label: localizations.popupMenuLabel,
-          child: Material(
-            type: MaterialType.transparency,
-            textStyle: route.style,
-            child: ScrollConfiguration(
-              // Dropdown menus should never overscroll or display an overscroll indicator.
-              // Scrollbars are built-in below.
-              // Platform must use Theme and ScrollPhysics must be Clamping.
-              behavior: ScrollConfiguration.of(context).copyWith(
-                scrollbars: false,
-                overscroll: false,
-                physics: const ClampingScrollPhysics(),
-                platform: Theme.of(context).platform,
-              ),
-              child: PrimaryScrollController(
-                controller: widget.route.scrollController!,
-                child: Scrollbar(
-                  thumbVisibility: true,
-                  child: ListView(
-                    padding: kMaterialListPadding,
-                    shrinkWrap: true,
-                    children: children,
+          child: ClipRRect(
+            borderRadius: widget.borderRadius ?? BorderRadius.zero,
+            clipBehavior: widget.borderRadius != null
+              ? Clip.antiAlias
+              : Clip.none,
+            child: Material(
+              type: MaterialType.transparency,
+              textStyle: route.style,
+              child: ScrollConfiguration(
+                // Dropdown menus should never overscroll or display an overscroll indicator.
+                // Scrollbars are built-in below.
+                // Platform must use Theme and ScrollPhysics must be Clamping.
+                behavior: ScrollConfiguration.of(context).copyWith(
+                  scrollbars: false,
+                  overscroll: false,
+                  physics: const ClampingScrollPhysics(),
+                  platform: Theme.of(context).platform,
+                ),
+                child: PrimaryScrollController(
+                  controller: widget.route.scrollController!,
+                  child: Scrollbar(
+                    thumbVisibility: true,
+                    child: ListView(
+                      padding: kMaterialListPadding,
+                      shrinkWrap: true,
+                      children: children,
+                    ),
                   ),
                 ),
               ),

packages/flutter/test/material/dropdown_test.dart

@@ -3809,4 +3809,35 @@ void main() {
     expect(tester.getBottomRight(find.text(hintText)).dx, 776.0);
     expect(tester.getBottomRight(find.text(hintText)).dy, 350.0);
   });
+
+  testWidgets('BorderRadius property clips dropdown menu', (WidgetTester tester) async {
+    const double radius = 20.0;
+
+    await tester.pumpWidget(
+      MaterialApp(
+        home: Scaffold(
+          body: Center(
+            child: DropdownButtonFormField<String>(
+              borderRadius: BorderRadius.circular(radius),
+              value: 'One',
+              items: <String>['One', 'Two', 'Three', 'Four']
+                .map<DropdownMenuItem<String>>((String value) {
+                  return DropdownMenuItem<String>(
+                    value: value,
+                    child: Text(value),
+                  );
+              }).toList(),
+              onChanged: (_) { },
+            ),
+          ),
+        ),
+      ),
+    );
+
+    await tester.tap(find.text('One'));
+    await tester.pumpAndSettle();
+
+    final RenderClipRRect renderClip = tester.allRenderObjects.whereType<RenderClipRRect>().first;
+    expect(renderClip.borderRadius, BorderRadius.circular(radius));
+  });
 }