ComboBoxItem to full width of ComboBox, when using SelectedIndex, or navigating keyboard?

Following XAML causes the "My stuff" being centered of ComboBox until I open the ComboBox, when it correctly stretches itself.

    <ComboBox Height="30" Width="300" HorizontalContentAlignment="Stretch" SelectedIndex="0">
        <ComboBoxItem HorizontalContentAlignment="Stretch">
            <Border Background="Red">
                <TextBlock>My stuff...</TextBlock>
            </Border>
        </ComboBoxItem>
    </ComboBox>

The question is, is it possible to get the ComboBoxItem being stretched even when it's selected using SelectedIndex? Same bug, or feature, happens if SelectedIndex is untouched (-1) and one selects the item using keyboard.

Workaround is probably to open the ComboBox programmatically in the beginning of app, which is rather ugly.

You just need to set the width of your border to the dynamic width of your outercontrol:

E.g.

Width="{Binding ElementName=combox1, Path=ActualWidth}">

Try this:

<ComboBox x:Name="combox1" Height="30" Width="300" HorizontalContentAlignment="Stretch" 
    SelectedIndex="0">
    <ComboBoxItem HorizontalContentAlignment="Stretch">
        <Border Background="Red" Width="{Binding ElementName=combox1, Path=ActualWidth}">
            <TextBlock>My stuff...</TextBlock>
        </Border>
    </ComboBoxItem>
</ComboBox>