Remove all background when running games in a windowed mode
Adjust the background color settings in your Steam window mode to make the surroundings black for a distraction-free experience.
The optimal choice is uncertain, but the fastest option is a black wallpaper with the taskbar concealed.
You can open a full-screen pixel test on a dead pixel site, hide the taskbar, and then press ALT-TAB to launch the game.
Thanks, I realized Steam might have a feature I overlooked.