public partial class PopupWindow : Window {

	public PopupWindow() {
		InitializeComponent();
		WindowStartupLocation = WindowStartupLocation.Manual;

		Loaded += (s, e) => { // HACK1: use Loaded
			Mouse.Capture(this); // HACK2: use Capture
			var p = PointToScreen(Mouse.GetPosition(this));
			Mouse.Capture(null);
			var source = PresentationSource.FromVisual(this);
			p = new Point( // HACK2: use Transform
				p.X / source.CompositionTarget.TransformToDevice.M11, 
				p.Y / source.CompositionTarget.TransformToDevice.M22
			);
			Left = p.X - (ActualWidth / 2);
			Top = p.Y - 16;
		};

	}
}