If your mouse is near the screen edges, it drifts in that direction. It's in the settings, game, rotate if cursor near edge

I am having the same issue. Just bought the game. It happens in both the 2.0 and 3.5 versions of the game and I have just a standard dell mouse that came with the computer.

Workarounds:
For 2.0, if I set the game to windowed mode it fixes it. But there is a crosshair now where the mouse is and the arrow/hand in the game will get out of sync with that. So a lot of movement in a scene will take my mouse out of the window and lose focus on the game. Kind of annoying.

For 3.5, the windowed mode trick doesn't work as the crosshair doesn't show up like it did in 2.0. For that, you still put it in windowed mode and just try not to touch the keyboard keys. Once the game loads, alt tab out and right back into the game. The mouse movement will be fine. If you ever have to touch the keyboard (i.e. press ctrl or type a message), the mouse goes back to drifting wildly around the screen. Alt tab out and back in again to fix. You have to do that everytime you touch a key on the keyboard. It sucks.

Seems like its tied to the keyboard instead of the mouse. Looking for a better fix now. Help?

@maxmad

It sounds like a different issue.

In the settings,ini set cursor = true;

This will use the windows cursor instead of the game cursor.

This may solve the issue.

That fixed it to an extent. Turning the cursor on via your method, gave me the crosshairs and the symptoms were just like the 2.0 version then. Turning the crosshairs on via the game option, did the same thing but I didn't have the annoying crosshairs. I still lose focus on the game window though because of it.

What is bugging me now is that I did something yesterday after posting my issue that fixed the unwanted mouse drifting in both versions. I can't recreate my fix though.

@maxmad

did you try a different keyboard / mouse? Or do you have any other input devices connected (trackpad maybe)?

I did happen to have a basic dell keyboard and basic logitech mouse stashed away. I tried changing out my basic dell mouse and logitech gaming keyboard with them, but it didn't fix it. I don't have any other devices plugged in either like a trackball or any other USB devices.

I'm going to keep tinkering but open to other ideas. The mouse was drifting wildly when I first got it, but then when I relaunched the game later that night, it was fine. Unfortunately it went back to being all wonky the next day. So just need to figure out the magic combination of whatever I did again...

*UPDATE*
Hmm so installed some windows updates and it is working again. Will be back if the problem comes back up.

Looks like a reboot and launch the game first thing fixes it every time for me.

Hmmm, sounds like a weird issue you are having.

Maybe you have some software installed that messes with the mouse somehow? (Desktop sharing, macro software comes to mind)

Did you try both the OpenGL and DirectX mode in XSP?

Yep so Rift nailed it. I use a program called synergy to share my mouse and keyboard across two PCs. When synergy is connected, I get the drifting mouse movement. Even if it was connected and loses connection, I get the drifting mouse. It doesn't drift when it never makes a connection with the second PC.

I can deal with that workaround fine. Hope this helps others having similar issues.