[BUG] kill-whole-line on folded subtrees [9.6.8 (release_9.6.8-3-g21171d @ /home/w/usr/emacs/0/29/0/lisp/org/)]

[Sebastian Miele]

In an emacs -Q, create an Org buffer with the following contents:

--8<---------------cut here---------------start------------->8---
* AB
** C
--8<---------------cut here---------------end--------------->8---

Fold the subtree under the heading AB, so that only a single line is
displayed (ending in "...").  With point between A and B, hit
C-S-<backspace> (kill-whole-line).

Expected: The whole _visible_ line, i.e., the entire contents of the
buffer is erased.  Actual behavior: The line with heading C remains.

Contrast this with the same experiment, except that the point is at the
beginning of the line containing AB when hitting C-S-<backspace>.  Then
the expected behavior happens.  According to the source of
kill-whole-line, the intended effect indeed is to kill a whole _visible_
line.

The following patch to the Emacs sources fixes the issue:

diff --git a/lisp/simple.el b/lisp/simple.el
index abd587245fe..44221f3fc24 100644
--- a/lisp/simple.el
+++ b/lisp/simple.el
@@ -6649,9 +6649,7 @@ kill-whole-line
                             (unless (bobp) (backward-char))
                             (point))))
        (t
-        (save-excursion
-          (kill-region (point) (progn (forward-visible-line 0) (point))))
-        (kill-region (point)
+         (kill-region (save-excursion (forward-visible-line 0) (point))
                      (progn (forward-visible-line arg) (point))))))
 
 (defun forward-visible-line (arg)

The reason for the issue probably is: Without the patch, the killing
happens in two stages.  The first kill-region kills from the beginning
of the line until after the A.  That kills the leading *.  That probably
somehow triggers Org visibility changes.  With the patch applied the
whole killing happens in one stage, probably without causing an
intermediate change of visibility.

I first reported this to bug-gnu-emacs@gnu.org (see
https://debbugs.gnu.org/65734).  However, Eli asks:

> I'm not sure I understand why this is deemed a problem in Emacs.
> Shouldn't Org redefine C-S-<backspace> if the default binding doesn't
> suit what happens in Org buffers?  Did you discuss this with Org
> developers?

Emacs  : GNU Emacs 29.1.50 (build 3, x86_64-pc-linux-gnu, GTK+ Version 3.24.38, 
cairo version 1.17.8)
 of 2023-09-04
Package: Org mode version 9.6.8 (release_9.6.8-3-g21171d @ 
/home/w/usr/emacs/0/29/0/lisp/org/)