Mouse selects everything on its own

Touch your screen to make it stop.

I have the same problem on a touchscreen machine. It just happened now, and I decided to see if someone offered a solution. @mnr's mention that it might be touchscreen-related made me try: Touching the screen made the mouse stop auto-highlighting.

Got to System Settings -> Universal Access -> Pointing and Clicking:

Make sure you have not activated Hover Click

There is a bug in X and it's already reported. Sadly it took a while for them to realize that it's made by X.