What version did you upgrade from?
What kind of mouse is it?
How is it attached to your computer?
What OS are you using?
Does AE CC 2014 behave properly?
I upgraded from CC2014 and that still works fine.
Its a standard 3-button microsoft wired mouse
I'm using El Capitain
My system is a 2.66-12 core with a dedicated Nvidia card
That's really odd. I don't recall seeing this issue mentioned on the forums before. There were some mouse issues recently, but they sounded quite different from yours and usually involved Windows machines (and, often Wacom tablets). If CC 2014 still works fine, that's pretty weird too. Did your mouse ever work correctly in any version of CC 2015?
Try the newest update to 13.7 and see if it works.
If it doesn't work at that point, try uninstalling CC 2015, running the Adobe CC Cleaner, restarting your machine, and reinstalling and updating CC 2015.