Able to set full screen width for MenuAnchor

### Steps to reproduce

MenuAnchor sets the full screen width, but it is not actually full screen.

### Expected results

MenuAnchor can set full screen width

### Actual results

MenuAnchor is not set to full screen width

### Code sample

Code sample

```dart
MenuAnchor(
childFocusNode: _buttonFocusNode,
menuChildren: [
MenuItemButton(
child: Text(MenuEntry.about.label),
onPressed: () => _activate(MenuEntry.about),
),
SizedBox(
width:MediaQuery.of(context).size.width,
child:Text("dsfdsf")
),


],
builder:
(BuildContext context, MenuController controller, Widget? child) {
return TextButton(
focusNode: _buttonFocusNode,
onPressed: () {
if (controller.isOpen) {
controller.close();
} else {
controller.open();
}
},
child: const Text('OPEN MENU'),
);
},
),
```

### Screenshots or Video


### Logs

Logs

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```

### Flutter Doctor output

Doctor output

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```