Zero width space shows as underscore

Question

Typing a zero-width space in TERMINAL emacs shows an underscore-ish character. How to make it showing nothing?

Answer 1

Refer to C-hig (elisp)Glyphless Chars

By default the zero-width characters are displayed using the thin-space display method which will "Display a thin space, 1-pixel wide on graphical displays, or 1-character wide on text terminals"; also noting that "The ‘thin-space’, ‘empty-box’, ‘hex-code’, and ASCII string display methods are drawn with the ‘glyphless-char’ face" which by default inherits from the underline face.

This default ensures that these (potentially-confusing) characters are not 100% invisible to the user, and can easily be made much more visible if desired; but there is also a zero-width display method which you can use to truly hide them from display:

(set-char-table-range glyphless-char-display
                      (char-from-name "ZERO WIDTH SPACE") 'zero-width)

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Personally I want these things to be as visible as possible -- I've encountered a situation where I was unwittingly copying and pasting zero-width joiners between visible characters, and I created a mess because I never knew they were there. To make sure they are apparent to me (as I don't expect such things to normally be in the text I edit), I use this:

(set-face-background 'glyphless-char "red")

(If you want to see how that looks, ‌c‍o‍p‍y​ ‌t‍h‍i‍s​ ‌s‍e‍n‍t‌e‍n‍c‍e​ ‌i‍n‍t‍o​ ‌E‍m‍a‍c‍s​ ‌a‍n‍d​ ‌c‍o‍n‍f‍i‍g‍u‍r‍e​ ‌t‍h‍a‍t​ ‌b‍a‍c‍k‍g‍r‍o‍u‍n‍d‍.)

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To hide such chars for a particular mode only, you could make the char table buffer-local in that mode.

;; Make glyphless chars highly visible.
(set-face-background 'glyphless-char "red")

;; Hide ZERO WIDTH chars completely in `org-mode'.
(add-hook 'org-mode-hook #'my-hide-zero-width-chars)

(defun my-hide-zero-width-chars ()
  "Make ZERO WIDTH characters completely invisible."
  (setq-local glyphless-char-display
              my-zero-width-glyphless-char-display))

(defvar my-zero-width-glyphless-char-display
  (let ((table (copy-sequence glyphless-char-display))
        (charnames (list "ZERO WIDTH SPACE"
                         "ZERO WIDTH NO-BREAK SPACE"
                         "ZERO WIDTH JOINER"
                         "ZERO WIDTH NON-JOINER")))
    (dolist (name charnames)
      (set-char-table-range table (char-from-name name) 'zero-width))
    table)
  "Variant of `glyphless-char-display' which hides ZERO WIDTH chars.")