30 minutes after I posted this, the Supreme Commander 2 game itself somehow got an update via Steam. Now it seems to run fine with proton experimental. So this issue can be deleted or closed.
Closing per the last comment.
Now I think all this was caused by confusion on my part regarding the Steam client global settings and game-level over-rides. Probably nothing to do with Proton. Closing. (er - I would have closed but kisak got there before me.)
proton experimentalx2 2024-03
Compatibility Report
System Information
I confirm:
proton log: https://gist.github.com/hfs2/7529caedf335d441387963fdc453efb8
proton log with proton 9.0beta - runs OK - for reference:
https://gist.github.com/hfs2/000fde527e523210c6cb1671290319c2
Symptoms
Game fails silently - never displays a window. The green Play button in the steam client flickers to blue Stop briefly (0.2 seconds?) then returns to green / Play. Command line output doesn't give me any clues.
https://gist.github.com/hfs2/75ed1d388b01e951238725dd7ddd6532
Reproduction
In the Steam client, go to properties for Supreme Commander 2
Force the use of Proton Experimental
The same game ran fine with the same settings on 2024-03-05 and maybe 03-06