bug#53398: 29.0.50; narrow-to-region induces buffer recentering

bug#53398: 29.0.50; narrow-to-region induces buffer recentering

[Jose A Ortega Ruiz]

Hi,

I've observed this behaviour in both emacs 28 and 29 when using
message-mode's message-kill-to-signature:

  - start composing a message, e.g. a reply, so that the current window
    shows only a portion of the buffer, and not the signature

  - with point somewhere on the lower half of the window, C-c C-z
    (message-kill-to-signature)

  - the text from point to signature is deleted alright, but the buffer
    contents is recentered, as if one had invoked recenter /twice/, so
    the window gets scrolled up.  previous behaviour was that the window
    wouldn't scroll at all, one would just see the contents of the
    buffer below point change (and show the signature).

The body of message-kill-to-signature is simple, and i've observed that
if one comments out the narrow-to-region invocation marked below, the
effect disappears:

       (defun message-kill-to-signature (&optional arg)
          "Kill all text up to the signature.
        If a numeric argument or prefix arg is given, leave that number
        of lines before the signature intact."
          (interactive "P" message-mode)
          (save-excursion
            (save-restriction
              (let ((point (point)))
        	(narrow-to-region point (point-max))  ;;;;;; <------------------------
        	(message-goto-signature)
        	(unless (eobp)
        	  (if (and arg (numberp arg))
        	      (forward-line (- -1 arg))
        	    (end-of-line -1)))
        	(unless (= point (point))
        	  (kill-region point (point))
        	  (unless (bolp)
        	    (insert "\n")))))))

so i am (wild) guessing that the spurious recenter might be related with
save-restriction workings in combination with narrow, but i don't really
know what i'm talking about :)

The only variable related to scroll i can see changed from its default
value in my config is scroll-preserve-screen-position, which i set to
'always, but changing its value doesn't affect the behaviour above in
any way.
            
Thanks,
jao


In GNU Emacs 29.0.50 (build 10, x86_64-pc-linux-gnu, cairo version 1.16.0)
 of 2022-01-20 built on rivendell
Repository revision: 172c055745b1eb32def7be8ddcaae975996a789f
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12014000
System Description: Debian GNU/Linux bookworm/sid

Configured using:
 'configure --prefix=/usr/local/stow/emacs --with-x-toolkit=no
 --with-imagemagick'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ
IMAGEMAGICK JPEG JSON LCMS2 LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2
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Load-path shadows:
/home/jao/lib/elisp/org-static-blog/org-static-blog hides /home/jao/.emacs.d/elpa.29/org-static-blog-20220110.739/org-static-blog
/home/jao/etc/emacs/site/custom hides /usr/local/stow/emacs/share/emacs/29.0.50/lisp/custom
/home/jao/.emacs.d/elpa.29/transient-20220117.1122/transient hides /usr/local/stow/emacs/share/emacs/29.0.50/lisp/transient

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-- 
He who joyfully marches to music in rank and file has already earned
my contempt. He has been given a large brain by mistake, since for him
the spinal cord would suffice.
 -Albert Einstein

bug#53398: 29.0.50; narrow-to-region induces buffer recentering

[Eli Zaretskii]

> From: Jose A Ortega Ruiz <jao@gnu.org>
> Date: Thu, 20 Jan 2022 20:59:25 +0000
> 
>        (defun message-kill-to-signature (&optional arg)
>           "Kill all text up to the signature.
>         If a numeric argument or prefix arg is given, leave that number
>         of lines before the signature intact."
>           (interactive "P" message-mode)
>           (save-excursion
>             (save-restriction
>               (let ((point (point)))
>         	(narrow-to-region point (point-max))  ;;;;;; <------------------------
>         	(message-goto-signature)
>         	(unless (eobp)
>         	  (if (and arg (numberp arg))
>         	      (forward-line (- -1 arg))
>         	    (end-of-line -1)))
>         	(unless (= point (point))
>         	  (kill-region point (point))
>         	  (unless (bolp)
>         	    (insert "\n")))))))
> 
> so i am (wild) guessing that the spurious recenter might be related with
> save-restriction workings in combination with narrow, but i don't really
> know what i'm talking about :)

Any chance of having a reproduction recipe that is simpler and
preferably doesn't include message.el at all?  IOW, if your guess is
correct, and the problem is inside narrow-to-region, we shouldn't need
this elaborate setup to show the issue, right?  Can you show such a
recipe?

Also, you say "previous behavior was", but when did you see that
previous behavior and in what Emacs version?

> The only variable related to scroll i can see changed from its default
> value in my config is scroll-preserve-screen-position, which i set to
> 'always, but changing its value doesn't affect the behaviour above in
> any way.

So you are saying that this recentering doesn't happen in "emacs -Q",
even if you do invoke that Message mode command?  If so, please try
to find which of your customizations do affect it.

bug#53398: 29.0.50; narrow-to-region induces buffer recentering

[Lars Ingebrigtsen]

Jose A Ortega Ruiz <jao@gnu.org> writes:

>   - start composing a message, e.g. a reply, so that the current window
>     shows only a portion of the buffer, and not the signature
>
>   - with point somewhere on the lower half of the window, C-c C-z
>     (message-kill-to-signature)
>
>   - the text from point to signature is deleted alright, but the buffer
>     contents is recentered, as if one had invoked recenter /twice/, so
>     the window gets scrolled up.  previous behaviour was that the window
>     wouldn't scroll at all, one would just see the contents of the
>     buffer below point change (and show the signature).

I've tried reproducing this in Emacs 29 with various contents in the
buffer and with various window sizes, but `C-c C-z' doesn't seem to do
any recentring for me.

Can you create a test recipe, starting from "emacs -Q", that
demonstrates the problem?

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

bug#53398: 29.0.50; narrow-to-region induces buffer recentering

[Jose A Ortega Ruiz]

On Fri, Jan 21 2022, Lars Ingebrigtsen wrote:

> Jose A Ortega Ruiz <jao@gnu.org> writes:
>
>>   - start composing a message, e.g. a reply, so that the current window
>>     shows only a portion of the buffer, and not the signature
>>
>>   - with point somewhere on the lower half of the window, C-c C-z
>>     (message-kill-to-signature)
>>
>>   - the text from point to signature is deleted alright, but the buffer
>>     contents is recentered, as if one had invoked recenter /twice/, so
>>     the window gets scrolled up.  previous behaviour was that the window
>>     wouldn't scroll at all, one would just see the contents of the
>>     buffer below point change (and show the signature).
>
> I've tried reproducing this in Emacs 29 with various contents in the
> buffer and with various window sizes, but `C-c C-z' doesn't seem to do
> any recentring for me.
>
> Can you create a test recipe, starting from "emacs -Q", that
> demonstrates the problem?

Yes, i finally managed to create a recipe.  It's kind of exotic and
involves setting a custom set-message-function that adds properties to
the displayed message (!).

More concretely, with emacs -Q, eval this code:

     (defun text-with-right-padding (text)
       (let* ((len (+ (string-pixel-width text) (string-pixel-width " ")))
              (padding (propertize "-"
                                   'display
                                   `(space :align-to (- (+ right right-margin) (,len))))))
         (concat padding text " ")))

     (defun custom-set-message (msg)
       (when msg (concat msg (text-with-right-padding (buffer-name)))))

     (setq set-message-function #'custom-set-message)

text-with-right-padding is copied directly from a message in emacs-devel
and i don't fully understand it (in particular, i cannot make sense of
the parens around (,len)), but it does its intended job: with
custom-set-message one sees messages displayed with the current buffer
name attached and padded to the right.

That triggers the recentering for me.  I just create a file with a bunch
of lines containing, e.g.

--8<---------------cut here---------------start------------->8---
slkdfjsldkfj
slkdfjsldkfj
slkdfjsldkfj
slkdfjsldkfj

slkdfjsldkfj
slkdfjsldkfj
slkdfjsldkfj
slkdfjsldkfj

;; ^^ repeat the above a bunch of times so that you don't
;; see the signature part below

-- 
slkdfjsldkfj
slkdfjsldkfj
slkdfjsldkfj
slkdfjsldkfj
--8<---------------cut here---------------end--------------->8---

Then load-library message, just move somewhere in the buffer without the
signature being visible and M-x message-kill-to-signature.

In my real case, i have a set-message-function that adds a varied amount
of extra status information aligned to the right.  I have a previous
version with adds the padding by computing manually the needed width and
simply concatenating in the middle with format and width spec, no fancy
'display properties.  With that function, the recentering does not
happen.

Hope this helps!

Cheers,
jao
-- 
What sane person could live in this world and not be crazy?
  -Ursula K. Le Guin, author

bug#53398: 29.0.50; narrow-to-region induces buffer recentering

[Eli Zaretskii]

> From: Jose A Ortega Ruiz <jao@gnu.org>
> Date: Sat, 22 Jan 2022 19:13:21 +0000
> Cc: 53398@debbugs.gnu.org
> 
>      (defun text-with-right-padding (text)
>        (let* ((len (+ (string-pixel-width text) (string-pixel-width " ")))
>               (padding (propertize "-"
>                                    'display
>                                    `(space :align-to (- (+ right right-margin) (,len))))))
>          (concat padding text " ")))
> 
>      (defun custom-set-message (msg)
>        (when msg (concat msg (text-with-right-padding (buffer-name)))))
> 
>      (setq set-message-function #'custom-set-message)
> 
> text-with-right-padding is copied directly from a message in emacs-devel
> and i don't fully understand it (in particular, i cannot make sense of
> the parens around (,len)), but it does its intended job: with
> custom-set-message one sees messages displayed with the current buffer
> name attached and padded to the right.

These customizations cause the echo-area message become too long for a
single screen line.  That in turn causes Emacs to resize the
mini-window to show the echo-area message, and that indirectly causes
the recentering you see.

So I don't think there's anything in this behavior that isn't
expected.  If you want to prevent recentering, try setting
max-mini-window-height to 1, so that mini-windows aren't resized.  (I
didn't try it, though.)  Or just don't ask the echo-area messages to
display so much stuff.

bug#53398: 29.0.50; narrow-to-region induces buffer recentering

[Lars Ingebrigtsen]

Eli Zaretskii <eliz@gnu.org> writes:

> So I don't think there's anything in this behavior that isn't
> expected.  If you want to prevent recentering, try setting
> max-mini-window-height to 1, so that mini-windows aren't resized.  (I
> didn't try it, though.)  Or just don't ask the echo-area messages to
> display so much stuff.

Yes, I don't think there's anything much we could do on the Emacs side
here -- any function that issues a message while narrowing is in effect
will lead to recentring with a set-message function like this.

So I'm closing this bug report.

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

bug#53398: 29.0.50; narrow-to-region induces buffer recentering

[Jose A Ortega Ruiz]

On Sun, Jan 23 2022, Lars Ingebrigtsen wrote:

> Eli Zaretskii <eliz@gnu.org> writes:
>
>> So I don't think there's anything in this behavior that isn't
>> expected.  If you want to prevent recentering, try setting
>> max-mini-window-height to 1, so that mini-windows aren't resized.  (I
>> didn't try it, though.)  Or just don't ask the echo-area messages to
>> display so much stuff.
>
> Yes, I don't think there's anything much we could do on the Emacs side
> here -- any function that issues a message while narrowing is in effect
> will lead to recentring with a set-message function like this.

just for my own understanding: are you saying that the fact that the
minibuffer displays more than one line causes a re-centering of my
current buffer? why is that normal? if i M-: (message "a\nb\n"), should
i expect that my current buffer is recentered?  (it isn't).

(also, for the record, the message displayed by that custom set-message
does not span more than one line, as far as i can tell.)

bug#53398: 29.0.50; narrow-to-region induces buffer recentering

[Lars Ingebrigtsen]

Jose A Ortega Ruiz <jao@gnu.org> writes:

> just for my own understanding: are you saying that the fact that the
> minibuffer displays more than one line causes a re-centering of my
> current buffer? why is that normal? if i M-: (message "a\nb\n"), should
> i expect that my current buffer is recentered?  (it isn't).

Causing such redisplay while the buffer is narrowed will have this effect.

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

bug#53398: 29.0.50; narrow-to-region induces buffer recentering

[Eli Zaretskii]

> From: Jose A Ortega Ruiz <jao@gnu.org>
> Cc: 53398@debbugs.gnu.org
> Date: Sun, 23 Jan 2022 15:42:51 +0000
> 
> just for my own understanding: are you saying that the fact that the
> minibuffer displays more than one line causes a re-centering of my
> current buffer? why is that normal? if i M-: (message "a\nb\n"), should
> i expect that my current buffer is recentered?  (it isn't).

If point is in the last N lines that would be "covered" by the
enlarged mini-window, then Emacs will recenter to keep point visible.

> (also, for the record, the message displayed by that custom set-message
> does not span more than one line, as far as i can tell.)

It does, see the continuation indicator at the right fringe.  You have
computed the alignment to span the entire screen line, but Emacs needs
one more place there to display the cursor.

bug#53398: 29.0.50; narrow-to-region induces buffer recentering

[Jose A Ortega Ruiz]

On Sun, Jan 23 2022, Eli Zaretskii wrote:

>> From: Jose A Ortega Ruiz <jao@gnu.org>
>> Cc: 53398@debbugs.gnu.org
>> Date: Sun, 23 Jan 2022 15:42:51 +0000
>> 
>> just for my own understanding: are you saying that the fact that the
>> minibuffer displays more than one line causes a re-centering of my
>> current buffer? why is that normal? if i M-: (message "a\nb\n"), should
>> i expect that my current buffer is recentered?  (it isn't).
>
> If point is in the last N lines that would be "covered" by the
> enlarged mini-window, then Emacs will recenter to keep point visible.
>
>> (also, for the record, the message displayed by that custom set-message
>> does not span more than one line, as far as i can tell.)
>
> It does, see the continuation indicator at the right fringe.  You have
> computed the alignment to span the entire screen line, but Emacs needs
> one more place there to display the cursor.

i see. thank you both Eli and Lars for your time, greatly appreciated.


cheers,
jao
-- 
Since when do we have to agree with people to defend them from
injustice? -Lillian Hellman, playwright (20 Jun 1905-1984)