Unlike native steam games, steam just appends the launch options after the target on non-steam games, even when %command% is added.
Hello @Unkn0wn-MDCLXIV, this is an issue with the Steam client, not Proton itself, and this issue is already being tracked at https://github.com/ValveSoftware/steam-for-linux/issues/6046. Closing as a duplicate.
Feature Request
I confirm:
contain this feature already.
Description
Unlike native steam games, steam just appends the launch options after the target on non-steam games, even when %command% is added.