PROTON_USE_WINED3D=1 launch option doesn't work in games when in game mode only in desktop mode

[carloslomas0]

Your system information

• Steam client version: 1785799196
• SteamOS version: 3.8.16
• Opted into Steam client beta?: No
• Opted into SteamOS beta?: No
• Have you checked for updates in Settings > System?: Yes

Please describe your issue in as much detail as possible:

There are some old games, like Fallout 4, where I still need to use the launch option PROTON_USE_WINED3D=1, but this option doesn't run in game mode only in desktop mode.

Steps for reproducing this issue:

1. Choose any game, released before 2016 if possible, and try to use PROTON_USE_WINED3D=1 %command% launch option in SteamOS game mode.
2. Game won't open.
3. Repeat the same in desktop mode, game will open.

[kisak-valve]

Hello @carloslomas0, while PROTON_USE_WINED3D can be useful for troubleshooting, it's not considered supported on Proton's side. Was there a separate issue with the game that led you towards tinkering in that direction? If so, please add a comment to ValveSoftware/Proton#308 with your out of the box experience.

Do you happen to have any OpenGL based game to use as an additional data point?