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

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

@@ -286,6 +286,11 @@ class _DropdownMenuState<T> extends State<_DropdownMenu<T>> {
           namesRoute: true,
           explicitChildNodes: true,
           label: localizations.popupMenuLabel,
+          child: ClipRRect(
+            borderRadius: widget.borderRadius ?? BorderRadius.zero,
+            clipBehavior: widget.borderRadius != null
+              ? Clip.antiAlias
+              : Clip.none,
             child: Material(
               type: MaterialType.transparency,
               textStyle: route.style,
@@ -314,6 +319,7 @@ class _DropdownMenuState<T> extends State<_DropdownMenu<T>> {
             ),
           ),
         ),
+      ),
     );
   }
 }

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));
+  });
 }