Hello. Thank you very much for reporting this issue, I didn't know the game is forced to run in full screen.
I apologize for having to go through all that (the command line arguments), I checked right now and if `Fullscreen` is unchecked here
Then the game will run windowed, but it won't be possible to select this while the demo popup shows up indeed.
It's strange that Unity doesn't handle this by itself, by creating a letterbox effect on ultrawide.
Do you see the game UI entirely stretched to the edges of the monitor? Or how was the "Next" button hidden?