New Post: ChildWindow in a UserControl

In my application I have a UserControl which contains a ChildWindow.
When I open the ChildWindow it gets centered inside the UserControl and can only moved inside it because it is part of its visual tree as far as I understood it.

But I want it to be centered and able to be moved over the whole application and not just the UserControl. Is there a way to accomplish this without implementing it outside of the UserControl?