Since I installed a graphics card, this * happens quite a lot. It seems to have various triggers, the easiest to reproduce is at the end of every video on youtube. Others may be related to idleing for some time or accidentally touching the mouse while watching.
And just be clear, this is not a laptop, there is only one screen connected, it never happened with integrated graphics.
*seems I’m not allowed to upload images or even links, so let me describe as best I can: A dark rectangle in the middle of the screen with the options to switch to external/internal monitor, the usual combinations of both and - my favourite - just click that distraction away.