Hello @Deus-nsf, we're using one issue report per unofficially supported game title, so I've gone ahead and transferred this issue report to https://github.com/ValveSoftware/Proton/issues/713#issuecomment-3225442220.
proton hotfixx1 2025-08PROTON_USE_WOW64=1x1 2025-08
I must clarify that I'm on the SteamDeck (even though that shouldn't really change much).
Reverting to Proton 8 fixes the issue, but more recent Proton are useful for some features, so it's a bit of a shame to have to resort to a downgrade. The issue seems to affect the way mouse control works for the game camera, it can also be reproduced on Proton 10 beta, Proton Exp, and Proton Hotfix, as well as many community branches that are based on 9 and up.
I have been hesitating on reporting this for a while as I thought it might be an issue on my end but it doesn't seem so, also more recent Proton are necessary for LSFG-VK to have PROTON_USE_WOW64=1 otherwise 32 bit apps won't work. And since OG Doom 3 is locked at 62.5 FPS, the newer features are appreciated.