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(elisp)Mouse Tracking

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Mouse Tracking
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   Sometimes it is useful to "track" the mouse, which means to display
something to indicate where the mouse is and move the indicator as the
mouse moves.  For efficient mouse tracking, you need a way to wait until
the mouse actually moves.

   The convenient way to track the mouse is to ask for events to
represent mouse motion.  Then you can wait for motion by waiting for an
event.  In addition, you can easily handle any other sorts of events
that may occur.  That is useful, because normally you don't want to
track the mouse forever--only until some other event, such as the
release of a button.

 - Special Form: track-mouse body...
     This special form executes BODY, with generation of mouse motion
     events enabled.  Typically BODY would use `read-event' to read the
     motion events and modify the display accordingly.  Note: Motion
     Events, for the format of mouse motion events.

     The value of `track-mouse' is that of the last form in BODY.  You
     should design BODY to return when it sees the up-event that
     indicates the release of the button, or whatever kind of event
     means it is time to stop tracking.

   The usual purpose of tracking mouse motion is to indicate on the
screen the consequences of pushing or releasing a button at the current
position.

   In many cases, you can avoid the need to track the mouse by using
the `mouse-face' text property (Note: Special Properties).  That
works at a much lower level and runs more smoothly than Lisp-level
mouse tracking.