Fix dropdown menu overscroll (#73654)

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

@@ -116,8 +116,8 @@ class _DropdownMenuItemButton<T> extends StatefulWidget {
 
   final _DropdownRoute<T> route;
   final EdgeInsets? padding;
-  final Rect? buttonRect;
-  final BoxConstraints? constraints;
+  final Rect buttonRect;
+  final BoxConstraints constraints;
   final int itemIndex;
 
   @override
@@ -138,7 +138,7 @@ class _DropdownMenuItemButtonState<T> extends State<_DropdownMenuItemButton<T>>
 
     if (focused && inTraditionalMode) {
       final _MenuLimits menuLimits = widget.route.getMenuLimits(
-          widget.buttonRect!, widget.constraints!.maxHeight, widget.itemIndex);
+          widget.buttonRect, widget.constraints.maxHeight, widget.itemIndex);
       widget.route.scrollController!.animateTo(
         menuLimits.scrollOffset,
         curve: Curves.easeInOut,
@@ -204,15 +204,15 @@ class _DropdownMenu<T> extends StatefulWidget {
     Key? key,
     this.padding,
     required this.route,
-    this.buttonRect,
-    this.constraints,
+    required this.buttonRect,
+    required this.constraints,
     this.dropdownColor,
   }) : super(key: key);
 
   final _DropdownRoute<T> route;
   final EdgeInsets? padding;
-  final Rect? buttonRect;
-  final BoxConstraints? constraints;
+  final Rect buttonRect;
+  final BoxConstraints constraints;
   final Color? dropdownColor;
 
   @override
@@ -288,12 +288,21 @@ class _DropdownMenuState<T> extends State<_DropdownMenu<T>> {
             textStyle: route.style,
             child: ScrollConfiguration(
               behavior: const _DropdownScrollBehavior(),
-              child: Scrollbar(
-                child: ListView(
-                  controller: widget.route.scrollController,
-                  padding: kMaterialListPadding,
-                  shrinkWrap: true,
-                  children: children,
+              child: PrimaryScrollController(
+                controller: widget.route.scrollController!,
+                child: LayoutBuilder(
+                  builder: (BuildContext context, BoxConstraints constraints) {
+                    final double menuTotalHeight = widget.route.itemHeights.reduce((double total, double height) => total + height);
+                    final bool isScrollable =  kMaterialListPadding.vertical + menuTotalHeight > constraints.maxHeight;
+                    return Scrollbar(
+                      isAlwaysShown: isScrollable,
+                      child: ListView(
+                        padding: kMaterialListPadding,
+                        shrinkWrap: true,
+                        children: children,
+                      ),
+                    );
+                  },
                 ),
               ),
             ),
@@ -512,14 +521,21 @@ class _DropdownRoute<T> extends PopupRoute<_DropdownRouteResult<T>> {
       menuTop = menuBottom - menuHeight;
     }
 
+    double scrollOffset = 0;
     // If all of the menu items will not fit within availableHeight then
     // compute the scroll offset that will line the selected menu item up
     // with the select item. This is only done when the menu is first
     // shown - subsequently we leave the scroll offset where the user left
     // it. This scroll offset is only accurate for fixed height menu items
     // (the default).
-    final double scrollOffset = preferredMenuHeight <= maxMenuHeight ? 0 :
-      math.max(0.0, selectedItemOffset - (buttonTop - menuTop));
+    if (preferredMenuHeight > maxMenuHeight) {
+      // The offset should be zero if the selected item is in view at the beginning
+      // of the menu. Otherwise, the scroll offset should center the item if possible.
+      scrollOffset = math.max(0.0, selectedItemOffset - (buttonTop - menuTop));
+      // If the selected item's scroll offset is greater than the maximum scroll offset,
+      // set it instead to the maximum allowed scroll offset.
+      scrollOffset = math.min(scrollOffset, preferredMenuHeight - menuHeight);
+    }
 
     return _MenuLimits(menuTop, menuBottom, menuHeight, scrollOffset);
   }

packages/flutter/test/material/dropdown_test.dart

@@ -991,6 +991,42 @@ void main() {
     );
   });
 
+  testWidgets('Dropdown menu scrolls to last item in long lists', (WidgetTester tester) async {
+    final Key buttonKey = UniqueKey();
+    await tester.pumpWidget(buildFrame(
+      buttonKey: buttonKey,
+      value: '99',
+      items: List<String>.generate(/*length=*/ 100, (int index) => index.toString()),
+      onChanged: onChanged,
+    ));
+    await tester.tap(find.byKey(buttonKey));
+    await tester.pump();
+
+    final ScrollController scrollController = PrimaryScrollController.of(tester.element(find.byType(ListView)))!;
+    // Make sure there is no overscroll
+    expect(scrollController.offset, scrollController.position.maxScrollExtent);
+
+    // Find the selected item in the scrollable dropdown list
+    final Finder menuItemFinder = find.byType(Scrollable);
+    final RenderBox menuItemContainer = tester.renderObject<RenderBox>(menuItemFinder);
+    final RenderBox selectedItem = tester.renderObject<RenderBox>(
+      find.descendant(
+        of: menuItemFinder,
+        matching: find.byKey(const ValueKey<String>('99')),
+      ),
+    );
+
+    // kMaterialListPadding.vertical is 8.
+    const Offset menuPaddingOffset = Offset(0.0, -8.0);
+    final Offset selectedItemOffset = selectedItem.localToGlobal(Offset.zero);
+    final Offset menuItemContainerOffset = menuItemContainer.localToGlobal(menuPaddingOffset);
+    // Selected item should be aligned to the bottom of the dropdown menu.
+    expect(
+      selectedItem.size.bottomCenter(selectedItemOffset).dy,
+      menuItemContainer.size.bottomCenter(menuItemContainerOffset).dy,
+    );
+  });
+
   testWidgets('Size of DropdownButton with null value', (WidgetTester tester) async {
     final Key buttonKey = UniqueKey();
     String? value;
@@ -1833,8 +1869,7 @@ void main() {
 
     double getMenuScroll() {
       double scrollPosition;
-      final ListView listView = tester.element(find.byType(ListView)).widget as ListView;
-      final ScrollController scrollController = listView.controller!;
+      final ScrollController scrollController = PrimaryScrollController.of(tester.element(find.byType(ListView)))!;
       assert(scrollController != null);
       scrollPosition = scrollController.position.pixels;
       assert(scrollPosition != null);
@@ -1870,8 +1905,7 @@ void main() {
 
     double getMenuScroll() {
       double scrollPosition;
-      final ListView listView = tester.element(find.byType(ListView)).widget as ListView;
-      final ScrollController scrollController = listView.controller!;
+      final ScrollController scrollController = PrimaryScrollController.of(tester.element(find.byType(ListView)))!;
       assert(scrollController != null);
       scrollPosition = scrollController.position.pixels;
       assert(scrollPosition != null);
@@ -1907,8 +1941,7 @@ void main() {
 
     double getMenuScroll() {
       double scrollPosition;
-      final ListView listView = tester.element(find.byType(ListView)).widget as ListView;
-      final ScrollController scrollController = listView.controller!;
+      final ScrollController scrollController = PrimaryScrollController.of(tester.element(find.byType(ListView)))!;
       assert(scrollController != null);
       scrollPosition = scrollController.position.pixels;
       assert(scrollPosition != null);
@@ -1944,8 +1977,7 @@ void main() {
 
     double getMenuScroll() {
       double scrollPosition;
-      final ListView listView = tester.element(find.byType(ListView)).widget as ListView;
-      final ScrollController scrollController = listView.controller!;
+      final ScrollController scrollController = PrimaryScrollController.of(tester.element(find.byType(ListView)))!;
       assert(scrollController != null);
       scrollPosition = scrollController.position.pixels;
       assert(scrollPosition != null);
@@ -2887,4 +2919,18 @@ void main() {
     expect(find.text('first').hitTestable(), findsNothing);
     expect(find.text('second').hitTestable(), findsNothing);
   });
+
+  testWidgets('Dropdown menu should persistently show a scrollbar if it is scrollable', (WidgetTester tester) async {
+    await tester.pumpWidget(buildFrame(
+      value: '0',
+      items: List<String>.generate(/*length=*/100, (int index) => index.toString()),
+      onChanged: onChanged,
+    ));
+    await tester.tap(find.text('0'));
+    await tester.pumpAndSettle();
+
+    final ScrollController scrollController = PrimaryScrollController.of(tester.element(find.byType(ListView)))!;
+    expect(scrollController.position.maxScrollExtent > 0, isTrue);
+    expect(find.byType(Scrollbar), paints..rect());
+  });
 }