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:
The second element of the list describes the current position of the mouse, just as in a click event (see Click Events).
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.