OK, it does sound a bit like it is screen resolution as it should be positioned at the bottom right of your screen as in my image.
There is actually no minimise button only a close button X only this is currently out of view. This scan in progress notification pop-up will have focus, in the same way as the task manager so you won't be able to access anything underneath/behind it.
However if you either click the avast tray icon or the Details button/link in the scan progress pop-up it will open the avastUI, Scan Computers section, once that is open you can minimise the avastUI to the tray
Whilst this isn't ideal it is al least a work around so you can access what is behind the scan progress pop-up.