Mouse selects everything on its own

Solution 1:

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.

Solution 2:

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

Make sure you have not activated Hover Click

Solution 3:

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.

Solution 4:

In my case, it was my mouse cord on top of the touchpad the one created the mayhem. It took me a shameful time to figure this out...