Title says it all. I'm not sure who thought this was a great idea, but now borderless windowed mode is absolutely broken if you have more than one monitor.

I'm really hoping this gets addressed and fixed immediately.

I have 3 and mine works fine, for the MOST part.
It is an issue with your video card more than the game.
It is due to the OS background being rendered and not "disabled" while you are controlling the game, and the mouse can slide out of the game and onto windows.

A trick to fix it is to move the windows tool bar to another location, them move it back.
Another is to change your in game video options to bordered, or fullscreen, then change it back.