Fix DropdownMenu with expandedInsets always aligned on top (#156214)
Fixes [DropdownMenu can not be center aligned when using expandedInsets
](https://github.com/flutter/flutter/issues/155581)
### Code sample
<details>
<summary>expand to view the code sample</summary>
```dart
import 'package:flutter/material.dart';
void main() => runApp(const MyApp());
class MyApp extends StatelessWidget {
const MyApp({super.key});
@override
Widget build(BuildContext context) {
final List<DropdownMenuEntry<ShortMenu>> shortMenuItems =
<DropdownMenuEntry<ShortMenu>>[];
for (final ShortMenu value in ShortMenu.values) {
final DropdownMenuEntry<ShortMenu> entry =
DropdownMenuEntry<ShortMenu>(value: value, label: value.label);
shortMenuItems.add(entry);
}
return MaterialApp(
home: Scaffold(
body: Row(
children: <Widget>[
Expanded(
child: Center(
child: DropdownMenu<ShortMenu>(
expandedInsets: const EdgeInsets.all(16),
initialSelection: ShortMenu.item0,
dropdownMenuEntries: shortMenuItems,
label: const Text('With expandedInsets'),
),
),
),
Expanded(
child: Center(
child: DropdownMenu<ShortMenu>(
initialSelection: ShortMenu.item0,
dropdownMenuEntries: shortMenuItems,
label: const Text('Without expandedInsets'),
),
),
),
],
),
),
);
}
}
enum ShortMenu {
item0('Menu 0'),
item1('Menu 1'),
item2('Menu 2');
const ShortMenu(this.label);
final String label;
}
```
</details>
### Before
(`DropdownMenu` without `expandedInsets` cannot be centered)
<img width="770" alt="Screenshot 2024-10-04 at 14 13 58" src="https://github.com/user-attachments/assets/c7520c12-d16a-4867-8fae-38b75dbc4225">
### After
(`DropdownMenu` with `expandedInsets` be centered)
<img width="770" alt="Screenshot 2024-10-04 at 14 13 49" src="https://github.com/user-attachments/assets/82e0b81e-5c85-4e59-99b8-df329459773b">
b3de00ad54