Upstream: https://github.com/ValveSoftware/Proton/
Write a logfile to $PROTON_LOG_DIR/steam-$APPID.log, where $PROTON_LOG_DIR defaults to your home directory.
PROTON_LOG=1 %command%
Log file with vulkan loader debug info:
VK_LOADER_DEBUG=all PROTON_LOG=1 %command%
ENABLE_VKBASALT=1 %command%
DXVK_HUD=1 %command%
VKD3D_DISABLE_EXTENSIONS=VK_KHR_present_id,VK_KHR_present_wait %command%
MANGOHUD=1 %command%
gamescope -f -H 1440 -- %command%
gamemoderun %command%