Update: If you're having mouse problems since this update, there's a new Beta to opt into. See here for the details.
A brand new update to Steam Play Proton has arrived ahead of the weekend with Proton 4.11-10, giving out of the box play for Halo: The Master Chief Collection.
For Halo, you will need the Steam Beta Client if you're on an older distribution, plus online matchmaking still won't work due to Easy Anti-Cheat not supporting Steam Play Proton. However, single-player does work fine and you should be able to play with friends outside of matchmaking. I've tested Halo myself, and it does work first time without issues signing into Live:
Also added in this release of Proton is a claim of major mouse handling improvements. They said this improved the experience in Fallout 4, Furi, and Metal Gear Solid V. On the subject of input there's also force feedback improvements, an Xbox controller performance regression was solved, and fixes for multiple games having problems with controller mapping like Telltale games with Xbox controllers and Cuphead and ICEY when using PlayStation 4 controllers over Bluetooth.
There's a new integer scaling mode available, which they said will "give sharp pixels when upscaling". You can enable it with the "WINE_FULLSCREEN_INTEGER_SCALING=1" environment variable.
Apart from that there's a fix for Metal Gear Solid V hanging on launch, Trine 4 should no longer be locked to 30 FPS and a frequent IL-2 Sturmovik crash should be solved. They also updated D9VK, FAudio and pulled in some misc DXVK bugfixes.
See the changelog here. To get the update, your Steam client should download it automatically. If not, ensure you have Proton 4.11 installed in the Tools menu.