ffected: experimental-11.0-20260713, hotfix-20260710. Not present in proton-10.0-4.

Change

Surfaced while playing EU5; 3450310

Proton 10.0 adds gamedrive to compat_config only inside per-appid allowlists (proton lines 1318, 1427, both indented within an if appid in [...]). Proton 11 replaces this with a single unconditional top-level call (Experimental line 1407, Hotfix line 1406):

  #options to also be enabled for prerequisite setup steps
  ret.add("gamedrive")

Every title now gets dosdevices/s: → /steamapps.

Problem

z: → / already exposes the entire filesystem, so for any game on a library outside the prefix, the library is now reachable through two drive letters simultaneously. Applications that enumerate paths under the Steam library see every file twice, as two unrelated locations:

S:\workshop\content<appid><id>
Z:\mnt\Steam\SteamLibrary\steamapps\workshop\content<appid><id>

Impact — Europa Universalis V (appid 3450310), and Paradox titles generally

1. Every Workshop mod appears twice in the in-game mod manager.
2. Worse: the engine mounts all installed Workshop items regardless of the active playset. logs/debug.log shows two Mounted Data: passes — an S:-prefixed pass covering everything installed, and a Z:-prefixed pass covering only the enabled playset.

Consequence (2) is a silent data-integrity problem. A mod I had explicitly disabled was loaded anyway; it renames a core game-database key, which orphaned other mods' references to it and destroyed player-built content mid-campaign. The mod was absent from the playset config and greyed out in the launcher, so it
took a long debugging session to identify. Any engine that treats "present in the library" as "available to load" will behave this way.

Reproduce

1. Install a Paradox title on a secondary Steam library (not the default under ~/.local/share/Steam), with several Workshop mods.
2. Proton 10.0 → dosdevices/ has only c: and z:; each mod listed once.
3. Proton Experimental/Hotfix → dosdevices/s: present while running; each mod listed twice.
4. grep "Mounted Data:" logs/debug.log → two mount passes, S: and Z:.

s: is created at launch and removed on exit, so inspect dosdevices/ while the game runs.