GNU bug report logs - #75042
29.4; `backward-delete-char-untabify': active-region behavior

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Asher Copeland <ashercopeland <at> gmail.com>
To: bug-gnu-emacs <at> gnu.org
Subject: 29.4; `backward-delete-char-untabify': active-region behavior
Date: Sun, 22 Dec 2024 17:52:04 -0800

The documentation of `backward-delete-char-untabify' indicates the
following:

 If Transient Mark mode is enabled, the mark is active, and ARG is 1,
 delete the text in the region and deactivate the mark instead.
 To disable this, set option `delete-active-region' to nil.

However, when `backward-delete-char-untabify-method' is set to hungry or
all and point has more than one whitespace character before it
(i.e. that `backward-delete-char-untabify' would delete more than one
character), `backward-delete-char-untabify' ignores the settings of
`transient-mark-mode' and `delete-active-region' and proceeds with its
normal action.

Practically, this means that, in a snippet of text such as the
following (assuming the spacing/newlines don't get messed up):

    line1
    line2
    line3

If point were placed before line2 and the region is active and nonempty,
with `backward-delete-char-untabify-method' set to hungry, the expected
behavior (and the behavior that is currently documented) would be that
the text highlighted by the region would be deleted and that the region
would be deactivated. What the function would instead do is delete the
spaces to the left of point and deactivate the region.

I have found this issue to occur running under `emacs -Q'.

For consistency with the current documentation and other deletion
commands, I believe that the behavior of `backward-delete-char-untabify'
should change to delete the text in the region, under the same
conditions that `backward-delete-char' would do the same.

Here is the last form of the definition of
`backward-delete-char-untabify', from simple.el:6522, for reference:

   (let* ((skip (cond ((eq backward-delete-char-untabify-method 'hungry) " \t")
                      ((eq backward-delete-char-untabify-method 'all)
                       " \t\n\r")))
          (n (if skip
                 (let* ((oldpt (point))
                        (wh (- oldpt (save-excursion
                                       (skip-chars-backward skip)
                                       (constrain-to-field nil oldpt)))))
                   (+ arg (if (zerop wh) 0 (1- wh))))
               arg)))
     ;; Avoid warning about delete-backward-char
     (with-no-warnings (delete-backward-char n killp))))

The issue seems to stem from the last line. In the described conditions,
the symbol `n' would be bound to a value greater than 1. Therefore,
`delete-backward-char' would delete n characters and deactivate the
region, instead of deleting the region.

--
Asher


In GNU Emacs 29.4 (build 1, x86_64-pc-linux-gnu, X toolkit, cairo
 version 1.18.0, Xaw scroll bars) of 2024-10-16 built on asher-v-ubuntu
System Description: Ubuntu 24.04.1 LTS

Configured using:
 'configure --with-x-toolkit=lucid --prefix=/opt/emacs'

Configured features:
CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG
LIBSELINUX LIBXML2 MODULES NOTIFY INOTIFY PDUMPER PNG RSVG SECCOMP SOUND
THREADS TIFF TOOLKIT_SCROLL_BARS TREE_SITTER WEBP X11 XDBE XIM XINPUT2
XPM LUCID ZLIB

Important settings:
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Major mode: Fundamental

Minor modes in effect:
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  tooltip-mode: t
  global-eldoc-mode: t
  show-paren-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
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Memory information:
((conses 16 38095 7179)
 (symbols 48 5293 1)
 (strings 32 13493 2007)
 (string-bytes 1 379725)
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 (vector-slots 8 105658 7932)
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Message #8 received at 75042 <at> debbugs.gnu.org (full text, mbox):

From: Asher Copeland <ashercopeland <at> gmail.com>
To: 75042 <at> debbugs.gnu.org
Subject: `backward-delete-char-untabify': active-region behavior
Date: Wed, 02 Apr 2025 10:59:04 -0700

[Message part 1 (text/plain, inline)]

Tags: patch

I have attached to this e-mail a patch that I believe fixes the bug, by
adding a condition that checks for the same conditions that
`delete-backward-char' uses for the same behavior, and delegates to
`delete-backward-char' if the conditions are met.

I have also found that the bug has another aspect, also fixed by the
patch. I will describe the steps for reproducing it below:

 1) emacs -Q
 2) C-q TAB w o r d
 3) C-SPC M-b
 4) Execute `backward-delete-char-untabify' (bound to DEL)

Notice that "word" was deleted, which is expected, but that the literal
TAB character was untabified, even though it was not necessary to do so.

The expected behavior is to delete "word" without untabifying.

I've looked at my previous e-mail and realized that I didn't leave clear
steps for reproduction for that aspect of the bug, so I will do so here:

 1) emacs -Q
 2) M-: (setq backward-delete-char-untabify-method 'hungry)
 3) SPC SPC w o r d
 4) C-SPC M-b
 5) Execute `backward-delete-char-untabify' (bound to DEL)

The expected behavior for it to delete "word", but instead it deletes
the two spaces to the left of point.

This patch fixes both issues with a single branch that makes it act like
`delete-backward-char' when it is appropriate to kill the region.

[0001-backward-delete-char-untabify-active-region-behavior.patch (text/patch, attachment)]

Message #15 received at 75042-done <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Asher Copeland <ashercopeland <at> gmail.com>
Cc: 75042-done <at> debbugs.gnu.org
Subject: Re: bug#75042: `backward-delete-char-untabify': active-region behavior
Date: Sun, 13 Apr 2025 11:27:12 +0300

> From: Asher Copeland <ashercopeland <at> gmail.com>
> Date: Wed, 02 Apr 2025 10:59:04 -0700
> 
> I have attached to this e-mail a patch that I believe fixes the bug, by
> adding a condition that checks for the same conditions that
> `delete-backward-char' uses for the same behavior, and delegates to
> `delete-backward-char' if the conditions are met.
> 
> I have also found that the bug has another aspect, also fixed by the
> patch. I will describe the steps for reproducing it below:
> 
>  1) emacs -Q
>  2) C-q TAB w o r d
>  3) C-SPC M-b
>  4) Execute `backward-delete-char-untabify' (bound to DEL)
> 
> Notice that "word" was deleted, which is expected, but that the literal
> TAB character was untabified, even though it was not necessary to do so.
> 
> The expected behavior is to delete "word" without untabifying.
> 
> I've looked at my previous e-mail and realized that I didn't leave clear
> steps for reproduction for that aspect of the bug, so I will do so here:
> 
>  1) emacs -Q
>  2) M-: (setq backward-delete-char-untabify-method 'hungry)
>  3) SPC SPC w o r d
>  4) C-SPC M-b
>  5) Execute `backward-delete-char-untabify' (bound to DEL)
> 
> The expected behavior for it to delete "word", but instead it deletes
> the two spaces to the left of point.
> 
> This patch fixes both issues with a single branch that makes it act like
> `delete-backward-char' when it is appropriate to kill the region.

Thanks, installed on the master branch, and closing the bug.

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