Emacs sometimes generates mouse motion events to describe motion of the mouse without any button activity. Mouse motion events are represented by lists that look like this:
position is a mouse position list (see Click Events), specifying the current position of the mouse cursor.
The special form
track-mouse enables generation of motion
events within its body. Outside of
track-mouse forms, Emacs
does not generate events for mere motion of the mouse, and these
events do not appear. See Mouse Tracking.