If you have used xinput to change your mouse’s coordinate transform matrix this may cause a problem with Steam Proton where the mouse gets stuck in the top-left corner. In a first person game this will manifest as the camera turning upwards and to the left (or downwards to the left if the mouse is inverted). The solution is to reset your coordinate transformation matrix to the default.
Run xinput list to see the list of devices, and note the name of the mouse device, for example Logitech USB Optical Mouse.
Run xinput list-props "Logitech USB Optical Mouse" to see the list of properties, and note the name of the coordinate tranformation matrix property, probably Coordinate Transformation Matrix.
Run
xinput set-prop "Logitech USB Optical Mouse" "Coordinate Transformation Matrix" 1 0 0 0 1 0 0 0 1to set the coordinate transformation matrix to its default value.