No matter what i tried. I could not figure out any reasonable way to center a Stage on Stage (for Dialog purposes), because either the border size of the Stage/Window is unknown or the size of the child Stage/Window is unknown before actually showing the child Stage on screen.

I could show the Stage first (now i know the sizes) and reposition it later, but that is a no go for any window manager that uses animations to show the window. It will break the animation and is not reliable.

This bugs me, because it is such an easy task in Swing and other Toolkits.