bug#45915: 28.2; delete-char deletes two letters

[Eli Zaretskii <eliz@gnu.org>]

> From: Tak Kunihiro <homeros.misasa@gmail.com>
> Cc: Tak Kunihiro <homeros.misasa@gmail.com>,  45915@debbugs.gnu.org,
>   yantar92@gmail.com
> Cc: tkk@misasa.okayama-u.ac.jp
> Date: Sat, 17 Sep 2022 17:20:26 +0900
> 
> 1. Following function will create a problematic org-table.
> 
> (defun emacs-bug-reproduce-45915 ()
>   "Reproduce bug#45915."
>   (interactive)
>   (with-current-buffer (get-buffer-create "*temp buffer*")
>     (erase-buffer)
>     (require 'org)
>     (orgtbl-mode 1)
>     (setq org-table-copy-increment nil)
>     (insert (format "emacs-version: %s, org-version: %s\n" emacs-version org-version))
>     (insert "| 1 |"))
>   (switch-to-buffer-other-window "*temp buffer*")
>   (execute-kbd-macro (kbd "M-< C-n C-f C-f"))
>   (message "I will call #'org-table-copy-down.")
>   (call-interactively #'org-table-copy-down))
> ;;; (call-interactively 'emacs-bug-reproduce-45915)
> 
> 2. Buffer is with the 1x2 table as shown below. <delete> deletes one
> letter on the first row, and it does two letters on the
> second row.
> 
>   emacs-version: 28.2, org-version: 9.5.5
>   | 1 |
>   | 1 |

No, it doesn't delete 2 characters in the 2nd line, it deletes just
one.  After you press <Delete>, type C-b (to go to the space between
the two '|' characters, and type "C-x =".  You will see this:

  Char: SPC (32, #o40, #x20, part of display "  ") point=51 of 54 (93%) column=1

Note the "51 to 54" part: there's a 'display' property there that
covers _two_ buffer positions, not one.  That's because the two
'display' properties on the two SPC characters before the deletion
have the same value:

>   (buffer-substring (line-beginning-position) (line-end-position))
> 
> line 1 returns: "| 1 |"
> line 2 returns: #("| 1 |" 1 2 (display (space :relative-width 1)) 3 4
> (display (space :relative-width 1)))

Those two 'display' properties are now displayed as a single stretch
glyph, because two adjacent text properties with the same value are
indistinguishable from a single one that spans all of the buffer
positions.

This is not a bug, this is how Emacs always worked.

The question is now why is this a problem for you, and how it affects
Org Table.  The solution, if there's a need for one, should IMO come
from the Org side.