Mouse cursor forced to top-left of screen.

[Zvi]

The video below shows it better than I can explain it, I think - that's just me trying to drag my mouse in various directions. It does this both at the login screen and in-game (which takes a lot of effort to get into, I might add!).

 

This does not occur on the Live client and only started after today's update to 27.4.4977.