-windowed in the steam launch argument. that's it, doing so will start the game as a window and then maximize it properly for it to run as intended

just make sure to NEVER EVER alt tab as it will break the window

another alternative solution you could try is temporarily setting your monitor resolution to 720p, and once your game has loaded alt+enter to make it a window and maximize it once you're back in 1080p, the issue is that it might not work for starters and if it does the window is extremely small with massive ugly black borders