## Description
This PR adds `DropdownMenu.decorationBuilder`.
The goal is to make `DropdownMenu` more flexible.
Before this PR, several fields are used by `DropdownMenu` to create an
inner `InputDecoration`. This approach has several limitations:
- `InputDecoration` has more fields that the ones that are exposed
- `DropdownMenu` makes some choices that can't be change. Especially, it
creates an IconButton (with hardcoded padding) which is passed to
`InputDecoration.suffixIcon`. This inner `IconButton` introduces some
difficulty related to focus management and UI customization.
The new `DropdownMenu.decorationBuilder` property offers users a way to
take control on the inner `InputDecoration` in a non-breaking way.
In a future PR, this property will help replacing the default
`IconButton`.
Currently users can replace the `IconButton` using this code sample:
<details><summary>DropdownMenu without IconButton</summary>
```dart
import 'package:flutter/material.dart';
void main() {
runApp(MyApp());
}
class MyApp extends StatelessWidget {
MyApp({super.key});
final List<DropdownMenuEntry<String>> menuEntries = [
"Red",
"Green",
"Blue",
].map((t) => DropdownMenuEntry<String>(label: t, value: t)).toList();
@override
Widget build(BuildContext context) {
return MaterialApp(
home: Scaffold(
body: Center(
child: SizedBox(
width: 220,
child: DropdownMenu<String>(
expandedInsets: EdgeInsets.zero,
requestFocusOnTap: true,
dropdownMenuEntries: menuEntries,
decorationBuilder: (context, controller) {
return InputDecoration(
labelText: 'Label text',
helperText: 'Select a color or enter one',
suffixIcon: controller.isOpen
? const Icon(Icons.arrow_drop_up)
: const Icon(Icons.arrow_drop_down),
);
},
),
),
),
),
);
}
}
```
</details>
## Related Issue
Fixes [DropDownMenu secondary trailing
widget](https://github.com/flutter/flutter/issues/175847)
Will help for [Make DropdownMenu's trailing icon not focusable by
default](https://github.com/flutter/flutter/issues/174096)
## Related discussions
https://github.com/flutter/flutter/issues/175847#issuecomment-3330098375https://github.com/flutter/flutter/pull/175558#discussion_r2380227394
## Tests
- Adds 7 tests.
6f743e3266