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So, I believe Subnautica is not a supported program but recently its actually been working relatively perfectly. If I alt-tab a few too many times it messes up but thats easily avoided by just not alt-tabbing while in game.

As of yesterday, however, keyboard input has stopped working. The mouse is present and functioning and my keyboard works outside of the game with no issues but in game, nothing. I can't even exit the game properly once in a save because I can't open the exit menu. (I also tried a new game just in case my old save was messed up somehow.)

I'm running it through Steam in a 2.3-staging drive with steam overlay and such turned off, if that makes any difference (don't see why it would though, the game worked fine the day before yesterday and I've not changed anything).