In my case, the error is extremely general, in that it happens apparently randomly during the use of any app, or even idle, with a slight bias toward it occurring while video is playing (either in browser or a media player). My laptop is very new, with a very new GPU, which might explain why Xid 109 is occurring so much for me despite not using Proton.
I had general big problems with Linux on this machine (which apparently is common for new hardware), most of which have been cleared up by BIOS updates. All that remains is lack of S3 suspend and this broken nVIdia driver.
I was hoping that my anomalous sample point might aid the devs in sniffing out a cause, but the radio silence continues…