DropdownMenu can be expanded to its parent size (#129753)
Fixes #125199
This PR is to add a new property `expandedInsets` so that the `DropdownMenu` can be expandable and has some margins around.
<details><summary>Example: Setting `expandedInsets` to `EdgeInsets.zero`</summary>
```dart
import 'package:flutter/material.dart';
void main() => runApp(const DropdownMenuExample());
class DropdownMenuExample extends StatefulWidget {
const DropdownMenuExample({super.key});
@override
State<DropdownMenuExample> createState() => _DropdownMenuExampleState();
}
class _DropdownMenuExampleState extends State<DropdownMenuExample> {
final TextEditingController colorController = TextEditingController();
ColorLabel? selectedColor;
@override
Widget build(BuildContext context) {
final List<DropdownMenuEntry<ColorLabel>> colorEntries = <DropdownMenuEntry<ColorLabel>>[];
for (final ColorLabel color in ColorLabel.values) {
colorEntries.add(
DropdownMenuEntry<ColorLabel>(value: color, label: color.label, enabled: color.label != 'Grey'),
);
}
return MaterialApp(
theme: ThemeData(
useMaterial3: true,
colorSchemeSeed: Colors.green,
),
home: Scaffold(
body: Center(
child: Container(
width: 500,
height: 500,
color: Colors.orange,
child: DropdownMenu<ColorLabel>(
expandedInsets: EdgeInsets.zero,
inputDecorationTheme: const InputDecorationTheme(
filled: true,
fillColor: Colors.white,
border: OutlineInputBorder(),
),
controller: colorController,
dropdownMenuEntries: colorEntries,
onSelected: (ColorLabel? color) {
setState(() {
selectedColor = color;
});
},
// expandedInsets: EdgeInsets.only(left: 35.0, right: 20.0, top: 80),
),
),
),
),
);
}
}
enum ColorLabel {
blue('Blue', Colors.blue),
pink('Pink', Colors.pink),
green('Green', Colors.green),
yellow('Yellow', Colors.yellow),
grey('Grey', Colors.grey);
const ColorLabel(this.label, this.color);
final String label;
final Color color;
}
```
<img width="500" alt="Screenshot 2023-06-28 at 11 33 20 PM" src="https://github.com/flutter/flutter/assets/36861262/e703f8a2-6e7c-45a0-86cf-d96da6dc157a">
</details>
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