WPF: Aligning the base line of a Label and its TextBox

This behaviour is, I think, caused by the fact that the TextBox defaults to a vertical alignment of Stretch, which causes it to fill the available space and have the extra couple of pixels under the text. If you use this instead:

<StackPanel>
    <StackPanel Orientation="Horizontal">
        <Label >MyLabel</Label>
        <TextBox VerticalAlignment="Center" Width="100">MyText</TextBox>
    </StackPanel>
</StackPanel>

… you should see a cleaner result.