System:

• AMD 9900X, MSI X870E,32gb RAM, 9070XT
  
• Tested with official builds of Ubuntu, CachyOS, and Bazzite (up-to-date for 12-12-2025)
  
• All partitions are formatted BTRFS

When multiple Linux user accounts share a single Steam Library folder (often located on a separate drive or partition) and attempt to play games utilizing Proton/Wine, a conflict occurs because the Wine prefix directory (located in steamapps/compatdata/) is implicitly shared by all users, regardless of drive/partition format.

My son and I have separate local user and Steam accounts on my desktop PC. We share a library via Steam Family Sharing - this shared library is in a dedicated partition (BTRFS) with proper permissions for us both to read, write, execute, and own all files and directories. This partition is set as the default library for Steam for both of us. However, whichever of us launches a shared game first, that individual will "own" the Wine Prefix folder in ~/compatdata/, causing the game to fail to launch for the other user.

• Wine prefixes must be exclusively owned by the single user running the application, and will fail to launch if permissions indicate shared or multi-user ownership.
  
• Steam stores game files and the corresponding Wine prefix (compatdata) within the same library structure. When a library is shared across multiple users, the compatdata directory is also shared, violating Wine's single-user requirement.
  
• Two or more Linux user accounts cannot launch the same Proton-enabled game from a Steam Library located on a drive or partition that is shared/mounted across all user accounts.
  
• When a game is launched from a shared drive with a prefix directory that is not owned by the current user, the game fails silently despite correct file permissions for the game files themselves.

This prevents multi-user gaming on Linux using a shared library, necessitating separate libraries in separate locations with redundant game installations.

The issue can be resolved by dynamically separating the Wine prefix from the shared Steam Library per user via a single symlink. My current workaround is a custom login script (bash) to manage the Steam Library directory per user on login:

1. Creates/verifies a user-specific compatdata directory in the user's home folder (e.g., ~/.local/share/steam-compatdata/).
   
2. Deletes the existing steamapps/compatdata symlink in the shared library.
   
3. Creates a new symbolic link from the shared library's steamapps/compatdata to the current user's isolated prefix folder (~/.local/share/steam-compatdata/).

Steam on Windows (understandably) does not have this issue as Wine prefixes are not needed, and separate Windows accounts handle the separation of game save data and dependencies. But these are bugs with Steam on Linux.

Allowing users to specify a local path where Proton should store compatdata for games in a shared library would maintain the efficiency of shared game files while respecting the single-user ownership requirement of Wine prefixes.