I have the problem aufm laptop too (win10) and do not know, obs is in this game mode. When I start the game it appears in the middle of the message. The position would just fit, as if that does not go away and creates that dead area. I am not sure though.
So I was right after all;)
So the credits go to you because I was reading about this bar during my Google searches. However, when I wrote my answer here, I could not exactly remember your message and where it was. So thank you for the help!