@TheHC From the app, if you click on your profile, there's an option for 'Display'. And it's basically a toggle for HDR. I'm assuming it's a general setting for all playing sessions, and not just for playing through phone. Just in case you're not aware and would like to try (I don't have HDR monitor)
Wonder what happens if 1 device supports HDR (for example my PC) and the other doesn't (my TV)
Don't have founders hardware delivered yet to try, but would be good to know.
@Professor78 : Currently it can only be managed via the Stadia app.
If you go to the Display settings for Stadia within Chrome, you are greeted with the message "Use the Stadia mobile app to manage these settings for now. They'll be available here soon."