Wenn ich programmgesteuert versuche, den Fokus zu verschieben, wird das Fokusvisual (das gepunktete Rechteck) nicht angezeigt.

Was kann getan werden, um diese visuelle Anzeige zu erzwingen?

<Window x:Class="WpfApplication2.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525" Loaded="OnLoaded">
    <StackPanel>
        <TextBlock x:Name="a" Focusable="True">A</TextBlock>
        <TextBlock Focusable="True">B</TextBlock>
        <Button Focusable="False" Click="OnClick">Move Focus</Button>
    </StackPanel>
</Window>

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
    }

    private void OnLoaded(object sender, RoutedEventArgs e)
    {
        Keyboard.Focus(a);
    }

    private void OnClick(object sender, RoutedEventArgs e)
    {
        var request = new TraversalRequest(FocusNavigationDirection.Next);
        var elementWithFocus = Keyboard.FocusedElement as UIElement;
        if (elementWithFocus != null)
            elementWithFocus.MoveFocus(request);
    }
}