Your system information

• Steam Client Build Date: Sep 17 2021, at 04:30:38
• Distribution: Manjaro Linux x86_64
• Opted into Steam client beta?: Yes
• Have you checked for system updates?: Yes

Please describe your issue in as much detail as possible:

I added a non-Steam Windows game, Tag: The Power of Paint to the Steam Linux client and I wanted to quantify the performance using MangoHUD. However I noticed that the %command% directive does not work when dealing with Non-Steam games and Proton. This makes it difficult to force things like Wine DLL Overrides, FSync, or ESync. With native Linux games it's not as big of a deal as you can make Steam point to a .sh script that will configure the game as necessary.

Steps for reproducing this issue:

1. Add Non-Steam Windows game
2. Add mangohud %command% to the launch options of the game.
3. Watch in awe as nothing happens relating to MangoHUD.