ProtonLauncher: empty UMU_ID to suppress GE-Proton X:=$HOME mount

Setting UMU_ID="fluorine" triggered protonfixes' setup_mount_drives
(GE-Proton/utilities.py), which mounts X:=$HOME, U:=/media, V:=/run/media,
W:=/mnt every launch — undoing pruneExtraDrives. Wine's find_drive_nt_root
walks the path innermost-out and picks the first matching drive, so
X:=$HOME wins over Z:=/ for any path under home, producing X:\games\...
launches.

Empty UMU_ID:
  - protonfixes utilities.py:38 `os.environ.get('UMU_ID','')` -> '' falsy
    -> setup_mount_drives early-exits, no X/U/V/W created
  - proton:1590 same falsy check -> 'gamedrive' not added to compat_config
    -> setup_game_dir_drive's setup_dir_drive takes the elif branch and
    actively removes any stale s: symlink
  - proton:2340 `"UMU_ID" in os.environ` is still True for "" -> still
    skips the c:\Program Files (x86)\Steam\steam.exe bridge for non-Steam
    executables (the original reason we set UMU_ID)

Verified against installed Proton-GE proton script + utilities.py and
GloriousEggroll's confirmation in umu-launcher#634.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

1 files changed, 9 insertions, 1 deletions

diff --git a/src/src/protonlauncher.cpp b/src/src/protonlauncher.cpp
index 8aecd47..d9b371a 100644
--- a/src/src/protonlauncher.cpp
+++ b/src/src/protonlauncher.cpp
@@ -668,8 +668,16 @@ bool ProtonLauncher::launchWithProton(qint64& pid) const
   // Proton-GE skips the built-in steam.exe bridge.  Without this, Proton
   // tries to initialise the Steam client which causes an assertion failure
   // for non-Steam executables.
+  //
+  // Use an EMPTY value, not "fluorine".  Proton's check is `"UMU_ID" in
+  // os.environ` (key presence) which is True for "" — so the steam.exe
+  // bridge is still skipped.  But protonfixes' setup_mount_drives keys on
+  // `os.environ.get('UMU_ID', '')` truthiness; with a non-empty value it
+  // mounts X:=$HOME, U:=/media, V:=/run/media, W:=/mnt and Wine then
+  // canonicalises game paths under X:\... instead of Z:\home\user\....
+  // Empty UMU_ID skips the mount block while preserving the bridge skip.
   if (!m_useSteamDrm) {
-    env.insert("UMU_ID", "fluorine");
+    env.insert("UMU_ID", "");
   }
 
   env.insert("DOTNET_ROOT", "");