I found the problem causing most, if not all of the mouse problems on LightDM: The Ambiance and/or Radiance themes. To fix the problem, install the Ubuntu Tweak PPA "ppa:tualatrix/ppa", install Ubuntu Tweak, and go to Ubuntu Tweak<Tweaks<Login Settings. Change the GTK theme to something else besides Ambiance or Radiance. A good, solid choice may be Adwaita.
I found the problem causing most, if not all of the mouse problems on LightDM: The Ambiance and/or Radiance themes. To fix the problem, install the Ubuntu Tweak PPA "ppa:tualatrix/ ppa", install Ubuntu Tweak, and go to Ubuntu Tweak<Tweaks<Login Settings. Change the GTK theme to something else besides Ambiance or Radiance. A good, solid choice may be Adwaita.
Hope this solved all of your problems :)