I have this same experience - no, the popup is not off the screen - it's too small and does not display the Cancel and Add to Allowlist buttons (nor is the scroll bar displayed, although I can scroll the list). I am able to end the CCleaner task using the task manager, but not otherwise. When I start CCleaner, the window is not full screen and that might be related to the faulty popup sizing. I put it to full screen just now and in that instance, I do see the buttons in the popup, so this is a bug unless someone thinks this sort of nonsense is a feature.