Add menuMaxHeight for DropdownButton (#76493)

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

@@ -325,7 +325,10 @@ class _DropdownMenuRouteLayout<T> extends SingleChildLayoutDelegate {
     // the view height. This ensures a tappable area outside of the simple menu
     // with which to dismiss the menu.
     //   -- https://material.io/design/components/menus.html#usage
-    final double maxHeight = math.max(0.0, constraints.maxHeight - 2 * _kMenuItemHeight);
+    double maxHeight = math.max(0.0, constraints.maxHeight - 2 * _kMenuItemHeight);
+    if (route.menuMaxHeight != null && route.menuMaxHeight! <= maxHeight) {
+      maxHeight = route.menuMaxHeight!;
+    }
     // The width of a menu should be at most the view width. This ensures that
     // the menu does not extend past the left and right edges of the screen.
     final double width = math.min(constraints.maxWidth, buttonRect.width);
@@ -411,6 +414,7 @@ class _DropdownRoute<T> extends PopupRoute<_DropdownRouteResult<T>> {
     this.barrierLabel,
     this.itemHeight,
     this.dropdownColor,
+    this.menuMaxHeight,
   }) : assert(style != null),
        itemHeights = List<double>.filled(items.length, itemHeight ?? kMinInteractiveDimension);
 
@@ -423,6 +427,8 @@ class _DropdownRoute<T> extends PopupRoute<_DropdownRouteResult<T>> {
   final TextStyle style;
   final double? itemHeight;
   final Color? dropdownColor;
+  final double? menuMaxHeight;
+
 
   final List<double> itemHeights;
   ScrollController? scrollController;
@@ -845,6 +851,7 @@ class DropdownButton<T> extends StatefulWidget {
     this.focusNode,
     this.autofocus = false,
     this.dropdownColor,
+    this.menuMaxHeight,
     // When adding new arguments, consider adding similar arguments to
     // DropdownButtonFormField.
   }) : assert(items == null || items.isEmpty || value == null ||
@@ -1094,6 +1101,17 @@ class DropdownButton<T> extends StatefulWidget {
   /// instead.
   final Color? dropdownColor;
 
+  /// The maximum height of the menu.
+  ///
+  /// The maximum height of the menu must be at least one row shorter than
+  /// the height of the app's view. This ensures that a tappable area
+  /// outside of the simple menu is present so the user can dismiss the menu.
+  ///
+  /// If this property is set above the maximum allowable height threshold
+  /// mentioned above, then the menu defaults to being padded at the top
+  /// and bottom of the menu by at one menu item's height.
+  final double? menuMaxHeight;
+
   @override
   _DropdownButtonState<T> createState() => _DropdownButtonState<T>();
 }
@@ -1240,6 +1258,7 @@ class _DropdownButtonState<T> extends State<DropdownButton<T>> with WidgetsBindi
       barrierLabel: MaterialLocalizations.of(context).modalBarrierDismissLabel,
       itemHeight: widget.itemHeight,
       dropdownColor: widget.dropdownColor,
+      menuMaxHeight: widget.menuMaxHeight,
     );
 
     navigator.push(_dropdownRoute!).then<void>((_DropdownRouteResult<T>? newValue) {
@@ -1488,6 +1507,7 @@ class DropdownButtonFormField<T> extends FormField<T> {
     )
     bool autovalidate = false,
     AutovalidateMode? autovalidateMode,
+    double? menuMaxHeight,
   }) : assert(items == null || items.isEmpty || value == null ||
               items.where((DropdownMenuItem<T> item) {
                 return item.value == value;
@@ -1556,6 +1576,7 @@ class DropdownButtonFormField<T> extends FormField<T> {
                      focusNode: focusNode,
                      autofocus: autofocus,
                      dropdownColor: dropdownColor,
+                     menuMaxHeight: menuMaxHeight,
                    ),
                  ),
                );

packages/flutter/test/material/dropdown_test.dart

@@ -53,6 +53,7 @@ Widget buildDropdown({
     bool autofocus = false,
     Color? focusColor,
     Color? dropdownColor,
+    double? menuMaxHeight,
   }) {
   final List<DropdownMenuItem<String>>? listItems = items?.map<DropdownMenuItem<String>>((String item) {
     return DropdownMenuItem<String>(
@@ -85,6 +86,7 @@ Widget buildDropdown({
         items: listItems,
         selectedItemBuilder: selectedItemBuilder,
         itemHeight: itemHeight,
+        menuMaxHeight: menuMaxHeight,
       ),
     );
   }
@@ -109,6 +111,7 @@ Widget buildDropdown({
     items: listItems,
     selectedItemBuilder: selectedItemBuilder,
     itemHeight: itemHeight,
+    menuMaxHeight: menuMaxHeight,
   );
 }
 
@@ -137,6 +140,7 @@ Widget buildFrame({
   Color? focusColor,
   Color? dropdownColor,
   bool isFormField = false,
+  double? menuMaxHeight,
 }) {
   return TestApp(
     textDirection: textDirection,
@@ -166,7 +170,9 @@ Widget buildFrame({
             dropdownColor: dropdownColor,
             items: items,
             selectedItemBuilder: selectedItemBuilder,
-            itemHeight: itemHeight,),
+            itemHeight: itemHeight,
+            menuMaxHeight: menuMaxHeight,
+          ),
         ),
       ),
     ),
@@ -2927,7 +2933,7 @@ void main() {
     expect(menuItemTapCounters, <int>[0, 2, 1, 0]);
   });
 
-  testWidgets('does not crash when option is selected without waiting for opening animation to complete', (WidgetTester tester) async {
+  testWidgets('Does not crash when option is selected without waiting for opening animation to complete', (WidgetTester tester) async {
     // Regression test for b/171846624.
 
     final List<String> options = <String>['first', 'second', 'third'];
@@ -3007,6 +3013,59 @@ void main() {
     expect(find.byType(Scrollbar), paints..rect());
   });
 
+  testWidgets("Dropdown menu's maximum height should be influenced by DropdownButton.menuMaxHeight.", (WidgetTester tester) async {
+    await tester.pumpWidget(buildFrame(
+      value: '0',
+      items: List<String>.generate(/*length=*/64, (int index) => index.toString()),
+      onChanged: onChanged,
+    ));
+    await tester.tap(find.text('0'));
+    await tester.pumpAndSettle();
+
+    final Element element = tester.element(find.byType(ListView));
+    double menuHeight = element.size!.height;
+    // The default maximum height should be one item height from the edge.
+    // https://material.io/design/components/menus.html#usage
+    final double mediaHeight = MediaQuery.of(element).size.height;
+    final double defaultMenuHeight = mediaHeight - (2 * kMinInteractiveDimension);
+    expect(menuHeight, defaultMenuHeight);
+
+    await tester.tap(find.text('0').last);
+    await tester.pumpAndSettle();
+
+    // Set menuMaxHeight which is less than defaultMenuHeight
+    await tester.pumpWidget(buildFrame(
+      value: '0',
+      items: List<String>.generate(/*length=*/64, (int index) => index.toString()),
+      onChanged: onChanged,
+      menuMaxHeight: 7 * kMinInteractiveDimension,
+    ));
+    await tester.tap(find.text('0'));
+    await tester.pumpAndSettle();
+
+    menuHeight = tester.element(find.byType(ListView)).size!.height;
+
+    expect(menuHeight == defaultMenuHeight, isFalse);
+    expect(menuHeight, kMinInteractiveDimension * 7);
+
+    await tester.tap(find.text('0').last);
+    await tester.pumpAndSettle();
+
+    // Set menuMaxHeight which is greater than defaultMenuHeight
+    await tester.pumpWidget(buildFrame(
+      value: '0',
+      items: List<String>.generate(/*length=*/64, (int index) => index.toString()),
+      onChanged: onChanged,
+      menuMaxHeight: mediaHeight,
+    ));
+
+    await tester.tap(find.text('0'));
+    await tester.pumpAndSettle();
+
+    menuHeight = tester.element(find.byType(ListView)).size!.height;
+    expect(menuHeight, defaultMenuHeight);
+  });
+
   // Regression test for https://github.com/flutter/flutter/issues/76614
   testWidgets('Do not crash if used in very short screen', (WidgetTester tester) async {
     // The default item height is 48.0 pixels and needs two items padding since