AFAIK, monitors themselves (nor gpus) can make expandable-by-mouse windows/dialog boxes... that has to be software of some sort... malware or something. Physical connections or riser cards, I can't imagine have anything to do with it. If it was an artifact, you couldn't manipulate it like that onscreen(?).
if you have a spare drive laying around, disconnect all your current drives and throw fresh windows on it. If it's gone, you know something on that install is the problem.
Or test the gpu in another pc...
...have you tried a driver wipe with ddu and use different drivers?