Oops you are right, try this:
Wire up the window to the following events (I did this via simple button click)
var childWindow = new ChildWindow();
childWindow.Closing += new EventHandler<CancelEventArgs>(OnChildWindowClosing);
childWindow.Show();
Now what you need to do is walk the ChildWindow PARTS DOM and find the ContentRoot which will give you the position.
static void OnChildWindowClosing(object sender, CancelEventArgs e)
{
var childWindow = (ChildWindow)sender;
var chrome = VisualTreeHelper.GetChild(childWindow, 0) as FrameworkElement;
if (chrome == null) return;
var contentRoot = chrome.FindName("ContentRoot") as FrameworkElement;
if (contentRoot == null || Application.Current == null || Application.Current.RootVisual == null) return;
var gt = contentRoot.TransformToVisual(Application.Current.RootVisual);
if (gt == null) return;
var windowPosition = gt.Transform(new Point(0, 0));
MessageBox.Show("X:" + windowPosition.X + " Y:" + windowPosition.Y);
}
HTH.