By background I mean whatever is not covered by any other object. In old posts someone mentioned something about window transparency object, however either I'm doing something wrong or it is not working as intended.

I need to develop several overlays and so on. Normally I'd use layers for that, however I need them to be scrollable and visible only in specific rect of the screen. I need also them to be interactable via mouse, so the viewport object is out of the question.