Welcome to ZDF!

I also now can't reproduce it as of 35471ce7e23494f37b879ffd8273603193118fdd, so I'd say this is all good now

I think it's gone for good now, I can no longer reproduce it in SW, Duke, and Blood as of 35471ce7e23494f37b879ffd8273603193118fdd , both when starting and quitting the game.

That line is the progress bar control from the startup screen. It has been a constant source of annoyance in GZDoom as well when the startup took a bit longer. I hope it's gone now but since I can'T see it on my setup I'd need confirmation that it's really gone now.

I think this started happening back when the migration to the new backend originally commenced.

I always see that line too at startup and when closing the game in Blood. It seems to be a more general issue rather than a game specific one, as it seems to happen mostly when going into fullscreen, but it happens in all games when quitting.

Haven't tested Blood on the new code but noticed it earlier in the week. Will backtrace it for you when I get a moment.