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14.15 Displaying Boundaries

Emacs can add an indicator to display a fill column position. The fill column indicator is a useful functionality especially in prog-mode to indicate the position of a specific column.

You can set the buffer-local variables display-fill-column-indicator and display-fill-column-indicator-character to activate the indicator and control how it looks, respectively.

Alternatively you can type M-x display-fill-column-indicator-mode or M-x global-display-fill-column-indicator-mode which enables the indicator locally or globally, respectively, and also chooses the character to use if none is already set. It is possible to use the first one to activate the indicator in a hook and the second one to enable it globally.

There are 2 buffer local variables and 1 face to customize this mode:

display-fill-column-indicator-column

Specifies the column number where the indicator should be set. It can take positive numerical values for the column or the special value t which means that the variable fill-column will be used.

Any other value disables the indicator. The default value is t.

display-fill-column-indicator-character

Specifies the character used for the indicator. This character can be any valid character including Unicode ones if the font supports them.

When the mode is enabled through the functions display-fill-column-indicator-mode or global-display-fill-column-indicator-mode, the initialization functions check if this variable is non-nil, otherwise the initialization tries to set it to U+2502 or ‘|’.

fill-column-indicator

Specifies the face used to display the indicator. It inherits its default values from the face shadow but without background color. To change the indicator color you need only set the foreground color of this face.

On graphical displays, Emacs can indicate the buffer boundaries in the fringes. If you enable this feature, the first line and the last line are marked with angle images in the fringes. This can be combined with up and down arrow images which say whether it is possible to scroll the window.

The buffer-local variable indicate-buffer-boundaries controls how the buffer boundaries and window scrolling is indicated in the fringes. If the value is left or right, both angle and arrow bitmaps are displayed in the left or right fringe, respectively.

If value is an alist (see Association Lists in the Emacs Lisp Reference Manual), each element (indicator . position) specifies the position of one of the indicators. The indicator must be one of top, bottom, up, down, or t which specifies the default position for the indicators not present in the alist. The position is one of left, right, or nil which specifies not to show this indicator.

For example, ((top . left) (t . right)) places the top angle bitmap in left fringe, the bottom angle bitmap in right fringe, and both arrow bitmaps in right fringe. To show just the angle bitmaps in the left fringe, but no arrow bitmaps, use ((top . left) (bottom . left)).

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