Randomly, the game will put itself into a small black window that shows nothing but still runs and I cannot get it to come back to full screen

Randomly the game will put itself into a small black rectangular window in the top left corner of my screen.

No manner of trying to bring to front, switch to, resize, etc., from the task manager, will bring the game back.

I can still hear the game going on in the background and I can still issue commands if I make OTC the active window.

Please advise.