I can see how this might be frustrating, but the overlay's intended use doesn't require it to open up out of the scroll area. It will always open on screen, regardless of how far down the page you have scrolled.
Overlays sort of open like a new artboard on top of the current one, so they will always open on the screen somewhere.
If you want to use the overlay as a tooltip, you can do that better with component states.
You can show us exactly what you want to use it for, and even share the file here (or via PM), and I'll see if I can help you set it up properly.