What is this and how get rid of it?

I accidentally get this up and I’m not sure what it is. I need to restart Unreal to get rid of it.

The grey box in your viewport? it is normally an indicator when you do drag drop with widgets which want to dock somewhere…it shouldn’t be visible after you release your mouse button. so a Editor restart fixed it or does it come back again?

Adnoh is right. it is a bug. I get this also from time to time. No need to restart, just close and reopen the viewport…