Solution:
public RadWindow()
{
InitializeComponent();
MouseRightButtonDown += BGWindow_MouseRightButtonDown;
Loaded += BGWindowLoaded;
}
private void BGWindowLoaded(object sender, RoutedEventArgs e)
{
var modalBackground = this.ParentOfType<Canvas>();
if (modalBackground != null)
modalBackground.MouseRightButtonDown += BGWindow_MouseRightButtonDown;
}
private void BGWindow_MouseRightButtonDown(object sender, MouseButtonEventArgs e)
{
e.Handled =
true;
}