- When the user does not specify the PROTONPATH, attempt to download the latest Proton if Proton cannot be found in either the Steam compat tools or local cache. Otherwise, prioritize referring to existing ones -- $HOME/.local/share/Steam/compatibilitytools.d and $HOME/.cache/ULWGL.
- Effectively, new installations will always download the latest Proton, while existing ones will simply be warned of a later version. Downloading is avoided if the latest version already exists in the cache, and the cache will be used as a last resort to set the variable. When we're unable to find an existing Proton or download one, we raise an error.
