I noticed that if I try to minimize it with the shortcut WIN+pag↓ when the cursor is hovering the window it fails to minimize as you all already said, but if I try to hit the same shortcut when the mouse is not hovering the opera window but anything else (also a plasma panel) it does minimize regulary... how strange

@stratorrider I already tried this but in my kubuntu 21.04 doesn't work.. the only way was to set focus rule on the entire system, but that was causing other problems to other apps

I noticed that if I try to minimize it with the shortcut WIN+pag↓ when the cursor is hovering the window it fails to minimize as you all already said, but if I try to hit the same shortcut when the mouse is not hovering the opera window but anything else (also a plasma panel) it does minimize regulary... how strange