Valve released a new Proton Experimental update today that adds in 1 new playable game, as well as a bunch of fixes for games like MGS V, Sackboy, Marvel Snap, and Halo Infinite:

• Now Playable: 雀姬 (Mahjong ladies).
• Fix Zeepkist freezing when using controller.
• Fix Microsoft Flight Simulator crashing during longer flights.
• Fix Vampire Survivors crashing on SteamOS 3.4.
• Enable hidraw for simucube racing wheels and Fanatec pedals.
• Fix Sackboy: A Big Adventure failing to start the first time it's launched.
• Fix Marvel Snap not being able connect to online services.
• Fix video playback speed/stutter in METAL GEAR SOLID V: THE PHANTOM PAIN.
• Fix Halo Infinite intro video playing in a wrong language.

You can view the full Proton Experimental changelog on Github.