Supporting vertical screen resolutions (GPD Win 2)

Report

System Information

• GPU: intel graphics
• Driver/LLVM version: Mesa 20.1.3
• Kernel version: 5.6.15-1-MANJARO
• Link to full system information report as Gist: https://gist.github.com/yossef-dev/fd69a7fe60176ae0c8a2d5400fc9e5a7
• Proton version: 5.0.9

Symptoms

I've got a very specific issue that's due to the nature of my device I think.
I'm playing proton games on my GPD Win 2 (a handheld gaming PC) and the display mounted horizontally is a vertical phone screen, so the true resolution is 720x1280 and not 1280x720 so I had to rotate the display under gnome settings so I can use the device comfortably.

The issue though is that on Proton games I can almost never change my resolution in-game, because I guess the resolutions with inverted dimensions are not whitelisted or something.

Somehow playing games in native res is not an issue (everything is rendered perfectly horizontally and the resolution shown is 1280x720 and not 720x1280), it's just that I can't switch resolution in games (it either doesn't offer any other resolution or it sometimes even crashes when I try to).

There are some exceptions like in games like Cuphead, where changing the resolution is done nicely though still reporting an inverted resolution compared to what the display actually uses (reporting 960x540 instead of 540x960 for example).

Why is it important?

Changing resolution is interesting especially on ultra mobile devices like the GPD Win 2 because lowering it can make drastic performance improvements. I was only able to do that on games offering changes with percentage resolution sliders (or by changing values on .ini config files).

I don't know if it's a trvial change to make or not but it would dramatically enhance my experience.

Start with the linked Gist and the reported GPD Win 2 setup, then reproduce resolution selection in Proton games with the display rotated. The issue names no source file, test, or entry point; done would mean rotated 720x1280 displays expose usable alternate resolutions and games can switch to them without crashing.