GNU bug report logs - #18412
24.3.93 regression: undo boundary inserted by delete-selection-mode (CUA)

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

From: David Reitter <david.reitter <at> gmail.com>
To: Bug-Gnu-Emacs <bug-gnu-emacs <at> gnu.org>
Subject: 24.3.93 regression: undo boundary inserted by delete-selection-mode
 (CUA)
Date: Fri, 5 Sep 2014 12:37:06 -0400

This is a regression in the 24.4 pretest compared to 24.3.

M-x cua-mode
(enter and) select a word “hello", e.g., via double-click
enter single char, e.g., “x”
C-_   (undo)



In Emacs 24.3, the undo will restore “hello”.

In Emacs 24.3.93 pretest, the undo will only delete “x”, but not restore “hello”.
An undo boundary is present in buffer-undo-list between the delete-selection step and the insertion of “x”.


The relevant change, I think, were changes to CUA on Dec 8 and 11, 2013:

commit f38bbfce43b11667ee1a09acf4f4f932bf7c9043
Author: Stefan Monnier <monnier <at> iro.umontreal.ca>
Date:   Sun Dec 8 01:24:54 2013 -0500

    Use delete-selection-mode in cya-mode.


As a fix...

Looking at how self-insert-command deletes the undo boundary to merge itself, we can either prevent the undo boundary from being inserted (I couldn’t figure out where that happens), or pretend that the delete-selection action was a self-insert-command.  The patch below is a proof of concept.

I think the right fix would be not to insert the undo boundary in the first place, if this is reasonably doable.





commit abf13771ad1b3dbc542a35393230eb43baa55d1c
Author: David Reitter <david.reitter <at> gmail.com>
Date:   Fri Sep 5 12:26:11 2014 -0400

    delsel: delete-active-region: avoid undo boundary
    
    Set last-command so that self-insert-command removes any
    undo boundary.



diff --git a/lisp/delsel.el b/lisp/delsel.el
index 1ada027..c1b1eba 100644
--- a/lisp/delsel.el
+++ b/lisp/delsel.el
@@ -80,6 +80,8 @@ If KILLP in not-nil, the active region is killed instead of deleted."
       (let (this-command)
 	(kill-region (point) (mark) t))
     (funcall region-extract-function 'delete-only))
+  ;; do not add undo-boundary in self-insert-command:
+  (setq last-command this-command)
   t)
 
 (defun delete-selection-helper (type)

Message #8 received at 18412 <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: David Reitter <david.reitter <at> gmail.com>
Cc: 18412 <at> debbugs.gnu.org
Subject: Re: bug#18412: 24.3.93 regression: undo boundary inserted by
 delete-selection-mode (CUA)
Date: Sun, 07 Sep 2014 22:04:29 -0400

> This is a regression in the 24.4 pretest compared to 24.3.
> M-x cua-mode
> (enter and) select a word “hello", e.g., via double-click
> enter single char, e.g., “x”
> C-_   (undo)
> In Emacs 24.3, the undo will restore “hello”.
> In Emacs 24.3.93 pretest, the undo will only delete “x”, but not
> restore “hello”.
> An undo boundary is present in buffer-undo-list between the
>  delete-selection step and the insertion of “x”.

Indeed, that's a difference between delete-selection-mode and cua's
earlier reimplementation of the feature.

To the extent that noone complained about it for delete-selection-mode,
I think this bug is not super-urgent to fix.  IOW, we should take the
time to think how to fix it right.

> Looking at how self-insert-command deletes the undo boundary to merge
> itself, we can either prevent the undo boundary from being inserted (I
> couldn’t figure out where that happens), or pretend that the
> delete-selection action was a self-insert-command.  The patch below is
> a proof of concept.

The undo-boundary is added by command_loop_1 in keyboard.c just before
calling command_execute.  Maybe the right fix is to change it so it's
push before running pre-command-hook?

Of course, maybe an even better fix is to change delete-selection-mode
so it doesn't rely on pre-command-hook.


        Stefan

Message #11 received at 18412 <at> debbugs.gnu.org (full text, mbox):

From: Lars Ingebrigtsen <larsi <at> gnus.org>
To: David Reitter <david.reitter <at> gmail.com>
Cc: 18412 <at> debbugs.gnu.org
Subject: Re: bug#18412: 24.3.93 regression: undo boundary inserted by
 delete-selection-mode (CUA)
Date: Wed, 09 Sep 2020 11:50:56 +0200

David Reitter <david.reitter <at> gmail.com> writes:

> This is a regression in the 24.4 pretest compared to 24.3.
>
> M-x cua-mode
> (enter and) select a word “hello", e.g., via double-click
> enter single char, e.g., “x”
> C-_   (undo)
>
> In Emacs 24.3, the undo will restore “hello”.
>
> In Emacs 24.3.93 pretest, the undo will only delete “x”, but not restore “hello”.

Stefan Monnier <monnier <at> iro.umontreal.ca> writes:

> Indeed, that's a difference between delete-selection-mode and cua's
> earlier reimplementation of the feature.
>
> To the extent that noone complained about it for delete-selection-mode,
> I think this bug is not super-urgent to fix.  IOW, we should take the
> time to think how to fix it right.

This was six years ago, and cua-mode still works the same way, so at
this point, I think changing the behaviour now would probably be a
regression?

So I'm closing this bug as a "wontfix".

-- 
(domestic pets only, the antidote for overdose, milk.)
   bloggy blog: http://lars.ingebrigtsen.no

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