How to change fringe background color for current line only?

Question

We can change current line background using global-hl-line-mode. How about fringe background of the current line?

Answer 1

The left and right fringe have one color that can be customized by the user; however, coloring only certain portions would need to be done with fringe bitmap images. Fringe bitmap images can be up to 8 pixels wide and up to frame-char-height in height. Emacs has about 10 to 15 built-in fringe bitmap images, and the user can customize his/her own. I like to use fringe-helper to create my own bitmap images https://github.com/nschum/fringe-helper.el. Emacs wiki has an example: https://www.emacswiki.org/emacs/FringeMark

Here is an example that runs every command loop on the post-command-hook that is local to the current-buffer only. You can use fringe-helper to create a rectangle the size and color of your liking.

(defface +-left-fringe-cursor-face
  '((t (:foreground "firebrick")))
  "Face for `+-left-fringe-cursor-face'."
  :group '+-mode)

(defun set-fringe-cursor ()
"Doc-string"
  (if (not (and (eobp) (bolp)))
     (setq +-left-fringe-overlay-position (copy-marker (line-beginning-position)))
    (setq +-left-fringe-overlay-position  nil)))

(define-fringe-bitmap '+-cursor-left-fringe-bitmap [128 192 96 48 24 48 96 192 128] 9 8 'center)
(set-fringe-bitmap-face '+-cursor-left-fringe-bitmap '+-left-fringe-cursor-face)

;;; `overlay-arrow-bitmap' is a special SYMBOL defined in xdisp.c.
(defvar +-left-fringe-overlay-position nil
  "Doc-string.")
(make-variable-buffer-local '+-left-fringe-overlay-position)
(add-to-list 'overlay-arrow-variable-list '+-left-fringe-overlay-position)
(put '+-left-fringe-overlay-position 'overlay-arrow-bitmap '+-cursor-left-fringe-bitmap)

(add-hook 'post-command-hook 'set-fringe-cursor 'append 'local)

---

Here is an example of a filled square bitmap image created using fringe-helper cited above, which is 8 pixels wide by 13 pixels high. [I just counted the number of x and see that it is only 7 and the pixel to the right is empty. The last pixel does not have to be empty -- I probably copied that bitmap from the Emacs source code. Feel free to place an x in lieu of the .]

;; AUTHOR:  Nikolaj Schumacher -- https://github.com/nschum/fringe-helper.el
(defun +-fringe-helper (&rest strings)
"Convert STRINGS into a vector usable for `define-fringe-bitmap'.
Each string in STRINGS represents a line of the fringe bitmap.
Periods (.) are background-colored pixel; Xs are foreground-colored. The
fringe bitmap always is aligned to the right. If the fringe has half
width, only the left 4 pixels of an 8 pixel bitmap will be shown.
For example, the following code defines a diagonal line.
\(+-fringe-helper
\"XX......\"
\"..XX....\"
\"....XX..\"
\"......XX\"\)"
  (unless (cdr strings)
    (setq strings (split-string (car strings) "\n")))
  (apply 'vector
    (mapcar
      (lambda (str)
        (let ((num 0))
          (dolist (c (string-to-list str))
            (setq num (+ (* num 2) (if (eq c ?.) 0 1))))
          num))
      strings)))

(define-fringe-bitmap '+-fringe-filled-rectangle (+-fringe-helper
"xxxxxxx."
"xxxxxxx."
"xxxxxxx."
"xxxxxxx."
"xxxxxxx."
"xxxxxxx."
"xxxxxxx."
"xxxxxxx."
"xxxxxxx."
"xxxxxxx."
"xxxxxxx."
"xxxxxxx."
"xxxxxxx.") nil nil 'center)

(defface +-fringe-filled-square-face
  '((t (:foreground "chartreuse")))
  "Face for `+-fringe-filled-square-face'."
  :group '+-mode)

(set-fringe-bitmap-face '+-fringe-filled-square '+-fringe-filled-square-face)

---

The built-in overflow cursor in fringe indicator (that appears if the user is on a line that is exactly the window-width) takes on the color of the cursor-color in the frame-parameters, and its image can be controlled as well. [See fringe-cursor-alist.] Other fringe bitmaps can be changed with fringe-indicator-alist.

Be aware that Emacs can also place fringe bitmap images using the built-in overlay mechanisms.

As indicated in a comment above, the user can also customize the line numbers to take on a foreground/background coloration of the user specifications -- this applies to the current line and to non-current lines.

Answer 2

Since Emacs 26.1, it is possible to enable display-line-numbers and configure line-number-current-line face.

Example configuration:

(setq-default display-line-numbers t)

(custom-theme-set-faces
 'misterioso
 '(line-number-current-line ((t (:background "SkyBlue4"))) t))

(enable-theme 'misterioso)