how to fix an overlay at end-of-line highlighting too much?

Question

I have added the following definitions to my initialization file in order to make the mark show up as a red highlight:

(defface mmv-face
  '((t :inherit default :background "red"))
  "Face used for showing the mark's position.")

(defvar mmv-mark-overlay nil "Overlay for showing the mark's position.")
(make-variable-buffer-local 'mmv-mark-overlay)

;; The three optional arguments are needed to cover the optional arguments of
;; those functions that this function will modify through `advice-add'.
(defun mmv-show-mark (&optional ARG1 ARG2 ARG3)
  "Make the mark's position stand out by means of a one-character-long overlay."
  (unless mmv-mark-overlay
    (setq mmv-mark-overlay (make-overlay 0 0 nil t))
    (overlay-put mmv-mark-overlay 'face 'mmv-face))
  (let ((mark-position (mark t)))
    (when mark-position
      ; Function `mark' returns non-nil only if the buffer's mark is set, so
      ; doing the following is pointless if `mark-position' is nil.
      (move-overlay mmv-mark-overlay mark-position (1+ mark-position)))))

(advice-add 'set-mark :after #'mmv-show-mark)
(advice-add 'pop-mark :after #'mmv-show-mark)
(advice-add 'push-mark :after #'mmv-show-mark)

It works well otherwise, except that if the mark is at the end of a line, everything to the right of the mark is also highlighted; see the screenshot below. Why would this be, and how can it be fixed?

Answer 1

When a face is placed on a newline (aka "LF") character, Emacs highlights the whole "rest of the line". That's why (in your case when the mark is at EOL, mark-position is just before LF and (1+ mark-position) is just after).

As for how to fix it, I think you'll want to do something like:

(defface mmv-face
  '((t :background "red"))
  "Face used for showing the mark's position.")

(defvar-local mmv-mark-overlay nil "Overlay for showing the mark's position.")

(defun mmv-show-mark (&rest _)
  "Make the mark's position stand out by means of a one-character-long overlay."
  (unless mmv-mark-overlay
    (setq mmv-mark-overlay (make-overlay 0 0 nil t))
    (overlay-put mmv-mark-overlay 'face 'mmv-face))
  (let ((mark-position (mark t)))
    (cond
     ((null mark-position) (delete-overlay mmv-mark-overlay))
     ((and (< mark-position (point-max))
           (not (eq ?\n (char-after mark-position))))
      (overlay-put mmv-mark-overlay 'after-string nil)
      (move-overlay mmv-mark-overlay
                    mark-position (1+ mark-position)))
     (t
      ;; This branch is called when the mark is at the end of a line or at the
      ;; end of the buffer. We use a bit of trickery to avoid the higlight
      ;; extending from the mark all the way to the right end of the frame.
      (overlay-put mmv-mark-overlay 'after-string
                   (propertize " " 'face (overlay-get mmv-mark-overlay 'face)))
      (move-overlay mmv-mark-overlay
                    mark-position mark-position)))))

(add-hook 'pre-redisplay-functions #'mmv-show-mark)

Note that this also takes care of removing the overlay when/if the mark is removed (this normally never happens, but we may as well), as well as handle the EOB case.

Two more things:

1. You can use defvar-local.
2. Rather than &optional ARG1 ARG2 ARG3 you're better doing &rest _ to simply accept any number of arguments and say that you'll ignore them.

3. Rather than advise an ever growing number of commands that can move/activate the mark, you should likely use

   (add-hook 'pre-redisplay-functions #'mmv-show-mark)
   

   this is the same hook normally used to highlight the region.