Vertical alignment of WPF Checkbox content with respect to checkbox

Question

I have a checkbox whose XAML markup is as follows:

<CheckBox HorizontalContentAlignment="Stretch">
    <StackPanel Orientation="Vertical">
        <TextBox x:Name="editTextBox" 
            Text="{Binding Path=Caption, Mode=TwoWay}"
            TextWrapping="Wrap"
            Visibility="{Binding Path=IsBeingEdited, Converter={StaticResource booleanToVisibilityConverter}}" />
        <TextBlock x:Name="itemText"
            TextWrapping="Wrap"                        
            Text="{Binding Path=Caption}" 
            Visibility="{Binding Path=IsBeingEdited, Converter={StaticResource reverseBooleanToVisibilityConverter}}">
        </TextBlock>
    </StackPanel>
</CheckBox>

The idea is that I can switch between the TextBlock (display) and TextBox (edit). However, when running the application, the CheckBox visual (the checkable square) is vertically centered. I would like it to be aligned with the top of the TextBlock/TextBox instead.

I've noticed that when I only include a TextBlock (so no StackPanel, no TextBox), the checkbox is in fact aligned with the top of the TextBlock, so I'm assuming I'm missing some setting on the StackPanel?