An extension of the echo area to display static messages (emacs)

echo-line.el

;; -------------------------------------------------------------------
;; An extension of the echo area to display static messages
;; Copyright 2020 Nicolas P. Rougier
;; -------------------------------------------------------------------
;; This file is not part of GNU Emacs.
;;
;; This program is free software: you can redistribute it and/or
;; modify it under the terms of the GNU General Public License as
;; published by the Free Software Foundation, either version 3 of the
;; License, or (at your option) any later version.
;;
;; This program is distributed in the hope that it will be useful, but
;; WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
;; General Public License for more details.
;;
;; You should have received a copy of the GNU General Public License
;; along with this program. If not, see <http://www.gnu.org/licenses/>
;; -------------------------------------------------------------------
(provide 'echo-line)

(require 'subr-x)

(defun echo-line-format ()
  "String to be appended at right of echo area."
  (propertize (format-mode-line "%l:%c") 'face 'face-faded))


(defun echo-line-message (orig-fun &rest args)
  "This enhanced message displays a regular message in the echo area
   and adds a specific text on the right part of the echo area. This
   is to be used as an advice."
  (let* ((right
          (concat
           ;; ! First space is a thin space, not a regular space
           ;; ! Last space needed to have truncated line
           " " (echo-line-format) " "
           ))
         (width (- (frame-width) (length right) 0))
         (msg (if (car args) (apply 'format-message args) ""))
         ;; Hack: The space for the split is a thin space, not a regular space
         ;; This way, we get rid of the added part if present (unless an actual
         ;; message uses a thin space.
         (msg (car (split-string msg " ")))
         (msg (string-trim msg))
         (left (truncate-string-to-width msg width nil nil "…"))
         (full (format (format "%%-%ds %%s" width) left right))
         )
    (if (active-minibuffer-window)
        ;; Regular log and display when minibuffer is active
        (apply orig-fun args)
      ;; Enhanced display
      (progn
        ;; Log actual message without echo
        (if message-log-max
            (let ((inhibit-message t)) (apply orig-fun (list msg))))
        ;; Display enhanced message without log
        (let ((message-truncate-lines t) (message-log-max nil))
          ;; (apply orig-fun (list full))
          (apply orig-fun (list (substring full 0 -1)))
         (set-display-table-slot
           (window-display-table (minibuffer-window))
           'truncation  (make-glyph-code (string-to-char (substring full -2))
                                         'face-faded))
          )
        ;; Set current message explicitely
        (setq current-message msg)))))


;; Install advice
(advice-add 'message :around #'echo-line-message)

;; Instal post-command hook
(add-hook 'post-command-hook
          (lambda () (let ((message-log-max nil))
                       (message (current-message)))))

;; Install a display table in minibuffer window
(set-window-display-table (minibuffer-window) (make-display-table))

The first backtrace happened when using dabbrev-expand and the second happened when using flyspell-buffer. Each command outputs a message that ends in %. I had run the code in the gist (without issue) before running those commands and running into the above errors.

Ok, thanks, I will try to debug it. In the menatime, you might be interested in mini-modeline

Thanks and thanks for the suggestion. I tried out mini-modeline, but decided not to use it because, as far as I could tell, it results in using minibuffer instead of the modeline, which isn't what I wanted; I wanted to use both the modeline and the minibuffer to display different information. I've found that I can do that with minibuffer-line, though.

I modified the code slightly to handle these (and other) issues. See here.