We exclusively reported that Google is working on bringing official Steam support to Chromebooks back in 2020. Parallel to developing a new Linux container better suited for games, the company is now additionally experimenting with a new "Game Mode" that could bring some further performance to the upcoming Linux container.
Chrome Unboxed found a new entry in Google's development resource Chromium Gerrit that hints at as much, aptly named "Toggle game mode for Borealis when full screen" — Borealis being the codename for the new Linux container. The entry further explains, "Implement logic to track window state and toggles game mode when a Borealis window is focused and enters and exits full screen." This indicates that the game mode will be turned on automatically whenever you open or close a game, and it's even possible that it won't even be a user-facing option that you can turn on and off. Instead, the game mode could be focused on automatically enabling things like pointer locks and performance prioritization, though at this point, that's just speculation.
After some more digging in the Gerrit, Chrome Unboxed found out that there will be clear messages when exiting and entering the game mode, potentially much like those you see when entering full-screen websites or a Stadia game.
Ultimately, this game mode is just another step of many to make gaming on Chromebooks better, and it could very well be a requirement before Google is ready to introduce Steam to the platform. The game mode is still in an early stage, so it'll take a while until we can tell for sure how exactly it will look like and what exactly it will do.