Dynamically set the Popup position/placement

Answer 1

I now realize that if I don't answer to my own question no one will be able to mark my work-around as an answer. So here's the full code. I implemented it as a PRISM behavior.

public static class PopupRightAlignBehavior
{
    public static readonly DependencyProperty InstanceProperty =
        DependencyProperty.RegisterAttached("Instance", typeof(object), typeof(PopupRightAlignBehavior), new PropertyMetadata(OnSetInstanceCallback));

    public static object GetInstance(DependencyObject obj)
    {
        return (object)obj.GetValue(InstanceProperty);
    }

    public static void SetInstance(DependencyObject obj, object value)
    {
        obj.SetValue(InstanceProperty, value);
    }

    private static void OnSetInstanceCallback(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var popup = (Popup)d;
        popup.Opened -= OnPopupOpened;
        popup.Opened += OnPopupOpened;
    }

    private static void OnPopupOpened(object sender, System.EventArgs e)
    {
        var popup = (Popup)sender;
        popup.Dispatcher.BeginInvoke(() => popup.HorizontalOffset = -popup.ActualWidth);
    }
}