The relative mouse mode broken in Proton 7.0.4 in several games for an actual external usb mouse on the Steam Deck, it affects Mass Effect Legendary Edition's mouse aim while walking around and Sims 3's photo mode (TAB key ingame while in live mode) and Skyrim SE's mouse aim (requires disabling gamepad controls in the settings) for example. Since the games otherwise work fine and most notably the mouse works fine in all menus and all other situations with absolute mode (other than Skyrim where it's also broken for some menus), and also the Steam Deck built-in inputs work fine, this seems to be a bug with Proton's treatment of relative mouse input.

Since this seems to be quite fundamental, a fix would be nice, it kind of makes FPS games unplayable if one doesn't want to use the Steam Deck controller inputs. (Assuming that it affects more games than those I tested.)

Steps to reproduce in Mass Effect Legendary Edition:

1. Boot a Steam Deck with default Steam OS with a standard usb mouse attached, non-beta, no desktop mode
2. Install ME:LE, set proton/steam play settings to default which at least for me resulted in Proton 7.0.4
3. Launch the game from Steam via "Play" button in the regular Deck gamepad ui and do the Origin account link if needed, then start a new game in Mass Effect 1. As soon as you're through the dialogue and cutscenes and can walk around, try to look around with the mouse

Expected behavior: works

Actual behavior: extreme drift in some random direction, it's impossible to use the mouse to look around. Which for an FPS game is pretty bad