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

Question

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.

Answer 1

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>