Sounds like a job for gamescope.
Linux
Everything about Linux
RULES
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Be nice to each other.
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No memes or pictures of Linux in the wild.
Doing this in reverse all the time. Stretching games on my triple head setup to full size. It's the same principle. If the game doesn't offer it kill it with gamescope, Wine virtual desktop or if nothing helps a completely virtual display that overlays your current:
https://beko.famkos.net/2021/05/31/extreme-multihead-for-gaming-on-linux-pc/
For some editing the config files directly works wonders. Looking at you Unity!
As the others said, windowed mode at your desired resolution, fullscreen at your desired resolution, or if neither work, gamescope.
Gamescope will make the game believe the display is a particular resolution and force it to run that way regardless of the actual hardware.
I don't recomend Ultrawide Displays for gaming on linux. Get it and you,LL SUFFER (spoky voice).. The EDID of my UW display (predator) is horrible .. like.. uw 1440p or 768p and nothing in between..
Tbh theres have been a couple of years since the last time I tried and people says that gamescope does the trick but I didn't tried.. but believe me that It was a real pain in the passt to avoid res stretching.