ZDoom

When alt tabbing with fullscreen turned on, the game will be on a complete black screen, you can still hear what's going on but for some reason the screen is completely black.

There are unfortunately not many things that can be done about this since this requires specific drivers and driver combinations to trigger. One workaround you can try is to alt-enter instead of alt-tab which puts GZDoom into a window while you work outside of GZDoom and then you can put it back to a window when you're done.

Roger that