elisp: Fringe Indicators
37.13.2 Fringe Indicators
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“Fringe indicators” are tiny icons displayed in the window fringe to
indicate truncated or continued lines, buffer boundaries, etc.
-- User Option: indicate-empty-lines
When this is non-‘nil’, Emacs displays a special glyph in the
fringe of each empty line at the end of the buffer, on graphical
displays. 
Fringes. This variable is automatically
buffer-local in every buffer.
-- User Option: indicate-buffer-boundaries
This buffer-local variable controls how the buffer boundaries and
window scrolling are indicated in the window fringes.
Emacs can indicate the buffer boundaries—that is, the first and
last line in the buffer—with angle icons when they appear on the
screen. In addition, Emacs can display an up-arrow in the fringe
to show that there is text above the screen, and a down-arrow to
show there is text below the screen.
There are three kinds of basic values:
‘nil’
Don’t display any of these fringe icons.
‘left’
Display the angle icons and arrows in the left fringe.
‘right’
Display the angle icons and arrows in the right fringe.
any non-alist
Display the angle icons in the left fringe and don’t display
the arrows.
Otherwise the value should be an alist that specifies which fringe
indicators to display and where. Each element of the alist should
have the form ‘(INDICATOR . POSITION)’. Here, INDICATOR is one of
‘top’, ‘bottom’, ‘up’, ‘down’, and ‘t’ (which covers all the icons
not yet specified), while POSITION is one of ‘left’, ‘right’ and
‘nil’.
For example, ‘((top . left) (t . right))’ places the top angle
bitmap in left fringe, and the bottom angle bitmap as well as both
arrow bitmaps in right fringe. To show the angle bitmaps in the
left fringe, and no arrow bitmaps, use ‘((top . left) (bottom .
left))’.
-- Variable: fringe-indicator-alist
This buffer-local variable specifies the mapping from logical
fringe indicators to the actual bitmaps displayed in the window
fringes. The value is an alist of elements ‘(INDICATOR .
BITMAPS)’, where INDICATOR specifies a logical indicator type and
BITMAPS specifies the fringe bitmaps to use for that indicator.
Each INDICATOR should be one of the following symbols:
‘truncation’, ‘continuation’.
Used for truncation and continuation lines.
‘up’, ‘down’, ‘top’, ‘bottom’, ‘top-bottom’
Used when ‘indicate-buffer-boundaries’ is non-‘nil’: ‘up’ and
‘down’ indicate a buffer boundary lying above or below the
window edge; ‘top’ and ‘bottom’ indicate the topmost and
bottommost buffer text line; and ‘top-bottom’ indicates where
there is just one line of text in the buffer.
‘empty-line’
Used to indicate empty lines when ‘indicate-empty-lines’ is
non-‘nil’.
‘overlay-arrow’
Used for overlay arrows (Overlay Arrow).
Each BITMAPS value may be a list of symbols ‘(LEFT RIGHT [LEFT1
RIGHT1])’. The LEFT and RIGHT symbols specify the bitmaps shown in
the left and/or right fringe, for the specific indicator. LEFT1
and RIGHT1 are specific to the ‘bottom’ and ‘top-bottom’
indicators, and are used to indicate that the last text line has no
final newline. Alternatively, BITMAPS may be a single symbol which
is used in both left and right fringes.
Fringe Bitmaps, for a list of standard bitmap symbols and
how to define your own. In addition, ‘nil’ represents the empty
bitmap (i.e., an indicator that is not shown).
When ‘fringe-indicator-alist’ has a buffer-local value, and there
is no bitmap defined for a logical indicator, or the bitmap is ‘t’,
the corresponding value from the default value of
‘fringe-indicator-alist’ is used.
Info Catalog
elisp: Fringe Size/Pos
elisp: Fringes
elisp: Fringe Cursors