Improve Dropdown Menu (#74906)

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

@@ -290,19 +290,13 @@ class _DropdownMenuState<T> extends State<_DropdownMenu<T>> {
               behavior: const _DropdownScrollBehavior(),
               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: Scrollbar(
+                  isAlwaysShown: true,
                   child: ListView(
                     padding: kMaterialListPadding,
                     shrinkWrap: true,
                     children: children,
                   ),
-                    );
-                  },
                 ),
               ),
             ),

packages/flutter/test/material/dropdown_test.dart

@@ -2923,13 +2923,31 @@ void main() {
   testWidgets('Dropdown menu should persistently show a scrollbar if it is scrollable', (WidgetTester tester) async {
     await tester.pumpWidget(buildFrame(
       value: '0',
+      // menu is short enough to fit onto the screen.
+      items: List<String>.generate(/*length=*/10, (int index) => index.toString()),
+      onChanged: onChanged,
+    ));
+    await tester.tap(find.text('0'));
+    await tester.pumpAndSettle();
+
+    ScrollController scrollController = PrimaryScrollController.of(tester.element(find.byType(ListView)))!;
+    // The scrollbar shouldn't show if the list fits into the screen.
+    expect(scrollController.position.maxScrollExtent, 0);
+    expect(find.byType(Scrollbar), isNot(paints..rect()));
+
+    await tester.tap(find.text('0').last);
+    await tester.pumpAndSettle();
+    await tester.pumpWidget(buildFrame(
+      value: '0',
+      // menu is too long to fit onto the screen.
       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)))!;
+    scrollController = PrimaryScrollController.of(tester.element(find.byType(ListView)))!;
+    // The scrollbar is shown when the list is longer than the height of the screen.
     expect(scrollController.position.maxScrollExtent > 0, isTrue);
     expect(find.byType(Scrollbar), paints..rect());
   });