Custom FormItem.Label cannot stretch to fit parent width in Form Container

[Flithor]

Describe the bug

As title

To Reproduce

<u:Form HorizontalAlignment="Stretch" VerticalAlignment="Top"
        LabelAlignment="Stretch" LabelPosition="Top">
    <TextBox>
        <u:FormItem.Label>
            <Grid>
                <TextBlock HorizontalAlignment="Left" Text="The Field" />
                <Button VerticalAlignment="Center" HorizontalAlignment="Right"
                        Padding="0" Content="BTN" />
            </Grid>
        </u:FormItem.Label>
    </TextBox>
</u:Form>

Expected behavior

No response

Avalonia version

11.3.2

OS

Windows

Additional context

Checking with F12, it seems that the problem is caused by multiple StackPanel nesting。

[timunie]

u:Form is nothing provided by Avalonia.

[Flithor]

Opps, I post it in wrong repository