Set the ScrollConfiguration for dropdown menus (#5255)

examples/flutter_gallery/flutter.yaml

@@ -25,7 +25,6 @@ assets:
   - packages/flutter_gallery_assets/pesto/image9.jpg
   - packages/flutter_gallery_assets/pesto/logo_small.png
   - packages/flutter_gallery_assets/pesto/logo_medium.png
-  - packages/flutter_gallery_assets/pesto/logo_big.png
   - packages/flutter_gallery_assets/pesto/fish.png
   - packages/flutter_gallery_assets/pesto/healthy.png
   - packages/flutter_gallery_assets/pesto/main.png

examples/flutter_gallery/lib/demo/pesto_demo.dart

@@ -12,8 +12,7 @@ const String _kUserImage = 'packages/flutter_gallery_assets/pesto/avatar.jpg';
 // Map of logo images keyed by the minimum height their container needs to be.
 final Map<double, String> _kLogoImages = <double, String>{
   0.0: 'packages/flutter_gallery_assets/pesto/logo_small.png',
-  70.0: 'packages/flutter_gallery_assets/pesto/logo_medium.png',
-  170.0: 'packages/flutter_gallery_assets/pesto/logo_big.png',
+  70.0: 'packages/flutter_gallery_assets/pesto/logo_medium.png'
 };
 
 final ThemeData _kTheme = new ThemeData(

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

@@ -75,6 +75,16 @@ class _DropDownMenuPainter extends CustomPainter {
   }
 }
 
+// Do not use the platform-specific default scroll configuration.
+// Dropdown menus should never overscroll or display an overscroll indicator.
+class _DropDownScrollConfigurationDelegate extends ScrollConfigurationDelegate {
+  const _DropDownScrollConfigurationDelegate();
+
+  @override
+  Widget wrapScrollWidget(Widget scrollWidget) => new ClampOverscrolls(value: true, child: scrollWidget);
+}
+final ScrollConfigurationDelegate _dropDownScroll = const _DropDownScrollConfigurationDelegate();
+
 class _DropDownMenu<T> extends StatefulWidget {
   _DropDownMenu({
     Key key,
@@ -159,6 +169,8 @@ class _DropDownMenuState<T> extends State<_DropDownMenu<T>> {
         child: new Material(
           type: MaterialType.transparency,
           textStyle: route.style,
+          child: new ScrollConfiguration(
+            delegate: _dropDownScroll,
             child: new Scrollbar(
               child: new ScrollableList(
                 scrollableKey: config.route.scrollableKey,
@@ -169,6 +181,7 @@ class _DropDownMenuState<T> extends State<_DropDownMenu<T>> {
             )
           )
         )
+      )
     );
   }
 }