The mouse cursor is drifting.
Start the game and the cursor will drift (including anywhere on the desktop if in windowed mode).
Update: I have unplugged my DualShock 4 controller and the problem went away. Replugged it and it came back. It seems to be related to the recent update on Steam Input.
proton experimental
Compatibility Report
System Information
I confirm:
Symptoms
The mouse cursor is drifting.
Reproduction
Start the game and the cursor will drift (including anywhere on the desktop if in windowed mode).