Subject: bug#79200: 31.0.50; Duplicated elements for '#<marker at' in buffer-undo-list

From: Eli Zaretskii <[email-masked]>
Date: Sat, 09 Aug 2025 16:51:03 +0300

> Cc: [email-masked], [email-masked], [email-masked],
>  [email-masked]
> From: Gerd Möllmann <[email-masked]>
> Date: Sat, 09 Aug 2025 15:33:56 +0200
> 
> Gerd Möllmann <[email-masked]> writes:
> 
> > Another thought: the format_mode_line_unwind is only called from
> > display_mode_line (redisplay) and format-mode-line. Why does soemthing
> > land in the undo list at all? DOes that make sense at all?
> 
> jit-lock uses this, which turns undo recording off
> 
> subr.el:
>  5253 (defmacro with-silent-modifications (&rest body)
>  5254   "Execute BODY, pretending it does not modify the buffer.
>  5255 This macro is typically used around modifications of
>  5256 text properties that do not really affect the buffer's content.
>  5257 If BODY performs real modifications to the buffer's text, other
>  5258 than cosmetic ones, undo data may become corrupted.
>  5259 
>  5260 This macro will run BODY normally, but doesn't count its buffer
>  5261 modifications as being buffer modifications.  This affects things
>  5262 like `buffer-modified-p', checking whether the file is locked by
>  5263 someone else, running buffer modification hooks, and other things
>  5264 of that nature."
>  5265   (declare (debug t) (indent 0))
>  5266   (let ((modified (make-symbol "modified")))
>  5267     `(let* ((,modified (buffer-modified-p))
>  5268             (buffer-undo-list t)
>                    ^^^^^^^^^^^^^^^^^^
> Maybe there specbinds missing somewhere.

I don't think so.  The additions to buffer-undo-list are because of
deleting some text, and that cannot be disabled.  The problem we see
is because there are many-many markers where the text is deleted.