Make Overlay size and position definable

I am trying to program a video wall where the content on the large monitor replicates a section of the smaller monitor from where the video wall is controlled. For this purpose I am replicating the contents using a ShaderEffectSource. Because this can only be used within one Window, the Window must stretch both screens.

If I open a dialog on the smaller monitor (e.g. because I need some input from the controller) both screens are dimmed by the Overlay, which I don't like. I would prefer to have the overlay only above the lower screen.

I have tried to set width and height of Overlay.overlay, but nothing seems to affect it. When I try to set it in Component.onCompleted Overlay.overlay is null, so I cannot assign any values. The same goes for Connections, where the target Overlay.overlay is null.