How to force windowpane to background without using the minimize-button?

[Leo]

Sounds like you've made the window on-top:

Why are my Opus windows on-top?

If you find your Opus windows are always on-top of other windows, even when the other windows are active, then at some point you probably accidentally activated the 'pin to top' behavior which causes this to happen. On-top Menu: The always-on-top fe…