A buffer can have blank areas called display margins on the left and on the right. Ordinary text never appears in these areas, but you can put things into the display margins using the display property. There is currently no way to make text or images in the margin mouse-sensitive.
The way to display something in the margins is to specify it in a margin display specification in the display property of some text. This is a replacing display specification, meaning that the text you put it on does not get displayed; the margin display appears, but that text does not.
A margin display specification looks like ((margin
right-margin) spec) or ((margin left-margin) spec). Here, spec is another display specification that says what to display in the margin. Typically it is a string of text to display, or an image descriptor.
To display something in the margin in association with certain buffer text, without altering or preventing the display of that text, put a before-string property on the text and put the margin display specification on the contents of the before-string.
Note that if the string to be displayed in the margin doesn’t specify a face, its face is determined using the same rules and priorities as it is for strings displayed in the text area (see Displaying Faces). If this results in undesirable “leaking” of faces into the margin, make sure the string has an explicit face specified for it.
Before the display margins can display anything, you must give them a nonzero width. The usual way to do that is to set these variables:
This variable specifies the width of the left margin, in character cell (a.k.a. “column”) units. It is buffer-local in all buffers. A value of nil means no left marginal area.
This variable specifies the width of the right margin, in character cell units. It is buffer-local in all buffers. A value of nil means no right marginal area.
Setting these variables does not immediately affect the window. These variables are checked when a new buffer is displayed in the window. Thus, you can make changes take effect by calling set-window-buffer. Do not use these variables to try to determine the current width of the left or right margin. Instead, use the function window-margins.
You can also set the margin widths immediately.
This function specifies the margin widths for window window, in character cell units. The argument left controls the left margin, and right controls the right margin (default 0).
If window is not large enough to accommodate margins of the desired width, this leaves the margins of window unchanged.
The values specified here may be later overridden by invoking set-window-buffer (see Buffers and Windows) on window with its keep-margins argument nil or omitted.
This function returns the width of the left and right margins of window as a cons cell of the form (left . right). If one of the two marginal areas does not exist, its width is returned as nil; if neither of the two margins exist, the function returns (nil). If window is nil, the selected window is used.
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Licensed under the GNU GPL license.
https://www.gnu.org/software/emacs/manual/html_node/elisp/Display-Margins.html