Add padding to DropdownButton (#115806)

* add padding param to DropdownButton

* improve padding comment

* update test

* Add more context to documentation

* update padding documentation with more detail

---------
Co-authored-by: Kate Lovett <katelovett@google.com>

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

@@ -886,6 +886,7 @@ class DropdownButton<T> extends StatefulWidget {
     this.enableFeedback,
     this.alignment = AlignmentDirectional.centerStart,
     this.borderRadius,
+    this.padding,
     // When adding new arguments, consider adding similar arguments to
     // DropdownButtonFormField.
   }) : assert(items == null || items.isEmpty || value == null ||
@@ -929,6 +930,7 @@ class DropdownButton<T> extends StatefulWidget {
     this.enableFeedback,
     this.alignment = AlignmentDirectional.centerStart,
     this.borderRadius,
+    this.padding,
     required InputDecoration inputDecoration,
     required bool isEmpty,
     required bool isFocused,
@@ -1115,6 +1117,17 @@ class DropdownButton<T> extends StatefulWidget {
   /// instead.
   final Color? dropdownColor;
 
+  /// Padding around the visible portion of the dropdown widget.
+  ///
+  /// As the padding increases, the size of the [DropdownButton] will also
+  /// increase. The padding is included in the clickable area of the dropdown
+  /// widget, so this can make the widget easier to click.
+  ///
+  /// Padding can be useful when used with a custom border. The clickable
+  /// area will stay flush with the border, as opposed to an external [Padding]
+  /// widget which will leave a non-clickable gap.
+  final EdgeInsetsGeometry? padding;
+
   /// The maximum height of the menu.
   ///
   /// The maximum height of the menu must be at least one row shorter than
@@ -1505,7 +1518,7 @@ class _DropdownButtonState<T> extends State<DropdownButton<T>> with WidgetsBindi
           autofocus: widget.autofocus,
           focusColor: widget.focusColor ?? Theme.of(context).focusColor,
           enableFeedback: false,
-          child: result,
+          child: widget.padding == null ? result : Padding(padding: widget.padding!, child: result),
         ),
       ),
     );
@@ -1566,6 +1579,7 @@ class DropdownButtonFormField<T> extends FormField<T> {
     bool? enableFeedback,
     AlignmentGeometry alignment = AlignmentDirectional.centerStart,
     BorderRadius? borderRadius,
+    EdgeInsetsGeometry? padding,
     // When adding new arguments, consider adding similar arguments to
     // DropdownButton.
   }) : assert(items == null || items.isEmpty || value == null ||
@@ -1635,6 +1649,7 @@ class DropdownButtonFormField<T> extends FormField<T> {
                    inputDecoration: effectiveDecoration.copyWith(errorText: field.errorText),
                    isEmpty: isEmpty,
                    isFocused: Focus.of(context).hasFocus,
+                   padding: padding,
                  ),
                );
              }),

packages/flutter/test/material/dropdown_test.dart

@@ -67,6 +67,7 @@ Widget buildDropdown({
     Color? focusColor,
     Color? dropdownColor,
     double? menuMaxHeight,
+    EdgeInsetsGeometry? padding,
   }) {
   final List<DropdownMenuItem<String>>? listItems = items?.map<DropdownMenuItem<String>>((String item) {
     return DropdownMenuItem<String>(
@@ -101,6 +102,7 @@ Widget buildDropdown({
         itemHeight: itemHeight,
         alignment: alignment,
         menuMaxHeight: menuMaxHeight,
+        padding: padding,
       ),
     );
   }
@@ -127,6 +129,7 @@ Widget buildDropdown({
     itemHeight: itemHeight,
     alignment: alignment,
     menuMaxHeight: menuMaxHeight,
+    padding: padding,
   );
 }
 
@@ -156,6 +159,7 @@ Widget buildFrame({
   Color? dropdownColor,
   bool isFormField = false,
   double? menuMaxHeight,
+  EdgeInsetsGeometry? padding,
   Alignment dropdownAlignment = Alignment.center,
 }) {
   return TestApp(
@@ -189,6 +193,7 @@ Widget buildFrame({
             itemHeight: itemHeight,
             alignment: alignment,
             menuMaxHeight: menuMaxHeight,
+            padding: padding,
           ),
         ),
       ),
@@ -3933,4 +3938,28 @@ void main() {
     final RenderClipRRect renderClip = tester.allRenderObjects.whereType<RenderClipRRect>().first;
     expect(renderClip.borderRadius, BorderRadius.circular(radius));
   });
+
+  testWidgets('Size of DropdownButton with padding', (WidgetTester tester) async {
+    const double padVertical = 5;
+    const double padHorizontal = 10;
+    final Key buttonKey = UniqueKey();
+    EdgeInsets? padding;
+
+    Widget build() => buildFrame(buttonKey: buttonKey, onChanged: onChanged, padding: padding);
+
+    await tester.pumpWidget(build());
+    final RenderBox buttonBoxNoPadding = tester.renderObject<RenderBox>(find.byKey(buttonKey));
+    assert(buttonBoxNoPadding.attached);
+    final Size noPaddingSize = Size.copy(buttonBoxNoPadding.size);
+
+    padding = const EdgeInsets.symmetric(vertical: padVertical, horizontal: padHorizontal);
+    await tester.pumpWidget(build());
+    final RenderBox buttonBoxPadded = tester.renderObject<RenderBox>(find.byKey(buttonKey));
+    assert(buttonBoxPadded.attached);
+    final Size paddedSize = Size.copy(buttonBoxPadded.size);
+
+    // dropdowns with padding should be that much larger than with no padding
+    expect(noPaddingSize.height, equals(paddedSize.height - padVertical * 2));
+    expect(noPaddingSize.width, equals(paddedSize.width - padHorizontal * 2));
+  });
 }