Adding my own report here since I shouldn't start a new issue, but have lots of additional problems and things to report.

Compatibility Report

• Name of the game with compatibility issues: Children of Morta
• Steam AppID of the game: https://steamdb.info/app/330020/

System Information

• GPU: Intel Mesa Intel UHD (CML GT2)
• Driver/LLVM version: 4.6 Mesa 20.0.5
• Kernel version: 5.6.12-arch1-1
• Link to full system information report: https://gist.github.com/WhyNotHugo/2374dd937039a655cf64f31e778002f6
• Proton version: 5.0-7

I confirm:

• [x] that I have checked whether there are updates for my system available.

steam-330020.log

Symptoms

• Running on my laptop's external display, it picked up the resolution of my laptop builtin display. The game rendered very small and covered the rest of the screen in black.
• Turning off the laptop display fixed this, however, mouse click on the menu didn't work. The game also didn't pick up my monitor's resolution, but stuck to just using the laptop's aspect ratio stretched out. Using the keyboard I could get into options, but then the keyboard can't be used to navigate further.
• Keyboard did work in all situations.
• I found that clicking on the far left of the screen resulted in the game interpreting clicks on the menu. Looks like the game has issues mapping click coordinates to screen coordinates.
• Running the game with only the [smaller] laptop screen did make the mouse work, so I could click around the menu. However, after staring a new game, I could just see a fixed background with a sky and stars. Lots of sound going on, but screen remained frozen.

I saw some references online that <Workaround removed by moderator> improved the experience. This was not the case for me.

Oh, sorry about that, I was unaware of that. It didn't fix anything anyway.