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. As with the end-position of a drag event, this position list may represent a location outside the boundaries of the initially selected frame, in which case the list contains that frame in place of a window.
track-mouse macro enables generation of motion
events within its body. Outside of
track-mouse body, Emacs
does not generate events for mere motion of the mouse, and these
events do not appear. See Mouse Tracking.
nil, mouse motion events are generated even for very
small movements. Otherwise, motion events are not generated as long
as the mouse cursor remains pointing to the same glyph in the text.