A weird problem started happening on my system in recent days (can't remember exactly). When I launch Activity Monitor, mouse pointer freezes and can't move normally until I kill Activity Monitor by using keyboard.
It works OK otherwise.
OS X version is
10.11.6 (15G1108). I have also installed SmoothMouse.
I removed SmoothMouse by issuing command
brew cask zap smoothmouse, now the mouse cursor can move while Activity Monitor is opened, but Activity Monitor itself is still freezing and is barely usable.