The overlay arrow is useful for directing the user’s attention to a particular line in a buffer. For example, in the modes used for interface to debuggers, the overlay arrow indicates the line of code about to be executed. This feature has nothing to do with overlays (see Overlays).
This variable holds the string to display to call attention to a
particular line, or nil if the arrow feature is not in use.
On a graphical display the contents of the string are ignored if the
left fringe is shown; instead a glyph is displayed in the fringe area
to the left of the display area.
This variable holds a marker that indicates where to display the overlay arrow. It should point at the beginning of a line. On a non-graphical display, or when the left fringe is not shown, the arrow text appears at the beginning of that line, overlaying any text that would otherwise appear. Since the arrow is usually short, and the line usually begins with indentation, normally nothing significant is overwritten.
The overlay-arrow string is displayed in any given buffer if the value
of overlay-arrow-position in that buffer points into that
buffer. Thus, it is possible to display multiple overlay arrow strings
by creating buffer-local bindings of overlay-arrow-position.
However, it is usually cleaner to use
overlay-arrow-variable-list to achieve this result.
You can do a similar job by creating an overlay with a
before-string property. See Overlay Properties.
You can define multiple overlay arrows via the variable
overlay-arrow-variable-list.
This variable’s value is a list of variables, each of which specifies
the position of an overlay arrow. The variable
overlay-arrow-position has its normal meaning because it is on
this list.
Each variable on this list can have properties
overlay-arrow-string and overlay-arrow-bitmap that
specify an overlay arrow string (for text terminals or graphical
terminals without the left fringe shown) or fringe bitmap
(for graphical terminals with a left fringe) to display at the
corresponding overlay arrow position. If either property is not set,
the default overlay-arrow-string or overlay-arrow fringe
indicator is used.