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Stay with experimental for now.
Do other games show a similar behavior or is it limited to KSP?
BG3 should work fine (was just playing it on Linux about 30m ago, but Arch, btw, etc).
You can get some extra logging from steam, if you exit completely and the, in a terminal, run:
steam -d
It’ll start Steam but output a lot of info to the terminal. The bit we’re interested in isn’t the stuff that it generates while Steam is starting. We want the bit that happens when you press play on a game. It’ll output the information about the important bits (like the Vulkan device, driver versions, monitors, etc )
Make sure there’s no obvious account info in the logs (there shouldn’t be, but always check) and post that.
I’m off to bed but I’ll check in with you tomorrow
No errors, that’s good and also not useful :/
As an aside, this is likely not the problem, but a good tip in general, is to use protonup to install GE-Proton (https://github.com/GloriousEggroll/proton-ge-custom). It is a community fork which essentially Proton Experimental + community fixes. The System76 article I linked above has the instructions (TL;DR, install protonup (terminal) or protonup-qt (GUI) and they’ll grab it for you and put it in the right directory, restart Steam and select the new version from the Compatibility menu either globally or per-game).
You essentially always want to be using the latest version of Proton unless something that was working breaks in a newer version.
So, next step, more logs:
You can enable proton logging by setting PROTON_LOG=1 as an environmental variable. You can do this per-game by right clicking a game -> Properties -> General and editing the launch options to say
Launch the game and let it crash or whatever. There will be a steam-$APPID.log in your home directory.