Subject: Re: bug#40338: 27.0.60; c-fill-paragraph and after-change-functions

From: Noam Postavsky <[email-masked]>
Date: Mon, 30 Mar 2020 21:56:56 -0400

Yuan Fu <[email-masked]> writes:

> (defun mytrack-hook (beg end len)
>   (if (> len 0)
>       ;; delete
>       (with-current-buffer trackbuf
>         (delete-region beg (+ beg len)))
>     ;; insert
>     (let ((content (buffer-substring beg end)))
>       (with-current-buffer trackbuf
>         (goto-char beg)
>         (insert content)))))

I'm not very familiar with either eglot or cc-mode, but I can say the
problem in your example lies with mytrack-hook: there can be kinds of
changes other than just pure delete or insert (replace, for example).
It can be fixed like this:

(defun mytrack-hook (beg end len)
  (when (> len 0)
    ;; Delete old text.
    (with-current-buffer "trackbuf"
      (delete-region beg (+ beg len))))
  ;; Insert new text.
  (let ((content (buffer-substring beg end)))
    (with-current-buffer "trackbuf"
      (goto-char beg)
      (insert content))))