If an extent has the highlight property set, it will be
highlighted when the mouse passes over it. Highlighting is accomplished
by merging the extent's face with the highlight face. The effect
is as if a pseudo-extent with the highlight face were inserted
after the extent in the display order (see section Extent Endpoints, display
order).
You can also explicitly cause an extent to be highlighted. Only one extent at a time can be highlighted in this fashion, and any other highlighted extent will be de-highlighted.
nil) or
de-highlights (if highlight-p is nil) extent, if
extent has the highlight property. (Nothing happens if
extent does not have the highlight property.)
highlight-extent but highlights
or de-highlights the extent regardless of whether it has the
highlight property.
If an extent has a keymap property, this keymap will be consulted
for mouse clicks on the extent and keypresses made while point is
within the extent. The behavior of mouse clicks and keystrokes not
defined in the keymap is as normal for the buffer.