GNU bug report logs - #76911
dedicated side windows lose their dedication upon reuse

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

From: Daniel Colascione <dancol <at> dancol.org>
To: bug-gnu-emacs <at> gnu.org
Subject: dedicated side windows lose their dedication upon reuse
Date: Mon, 10 Mar 2025 00:58:53 -0700

Consider the code below:

    (let ((b1 (get-buffer-create "*b1*"))
          (b2 (get-buffer-create "*b2*"))
          (action '(display-buffer-in-side-window
                    (dedicated . t)
                    (side . left))))
      (display-buffer b1 action)
      (sit-for 1)
      (display-buffer b2 action))

It puts b1 in a side window, then reuses that side window for b2.
When we display b2, the window loses the strong dedication it had to b1
despite the display action for b2 including a request for a
dedicated window.

I'd expect the window to be dedicated to b2 at the end of the
above snippet.

Message #8 received at 76911 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Daniel Colascione <dancol <at> dancol.org>, martin rudalics <rudalics <at> gmx.at>
Cc: 76911 <at> debbugs.gnu.org
Subject: Re: bug#76911: dedicated side windows lose their dedication upon reuse
Date: Mon, 10 Mar 2025 18:36:47 +0200

> From: Daniel Colascione <dancol <at> dancol.org>
> Date: Mon, 10 Mar 2025 00:58:53 -0700
> 
> 
> Consider the code below:
> 
>     (let ((b1 (get-buffer-create "*b1*"))
>           (b2 (get-buffer-create "*b2*"))
>           (action '(display-buffer-in-side-window
>                     (dedicated . t)
>                     (side . left))))
>       (display-buffer b1 action)
>       (sit-for 1)
>       (display-buffer b2 action))
> 
> It puts b1 in a side window, then reuses that side window for b2.
> When we display b2, the window loses the strong dedication it had to b1
> despite the display action for b2 including a request for a
> dedicated window.
> 
> I'd expect the window to be dedicated to b2 at the end of the
> above snippet.

Martin, any comments?

Message #11 received at 76911 <at> debbugs.gnu.org (full text, mbox):

From: martin rudalics <rudalics <at> gmx.at>
To: Eli Zaretskii <eliz <at> gnu.org>, Daniel Colascione <dancol <at> dancol.org>
Cc: 76911 <at> debbugs.gnu.org
Subject: Re: bug#76911: dedicated side windows lose their dedication upon reuse
Date: Mon, 10 Mar 2025 18:47:21 +0100

>> Consider the code below:
>>
>>      (let ((b1 (get-buffer-create "*b1*"))
>>            (b2 (get-buffer-create "*b2*"))
>>            (action '(display-buffer-in-side-window
>>                      (dedicated . t)
>>                      (side . left))))
>>        (display-buffer b1 action)
>>        (sit-for 1)
>>        (display-buffer b2 action))
>>
>> It puts b1 in a side window, then reuses that side window for b2.
>> When we display b2, the window loses the strong dedication it had to b1
>> despite the display action for b2 including a request for a
>> dedicated window.
>>
>> I'd expect the window to be dedicated to b2 at the end of the
>> above snippet.
>
> Martin, any comments?

The following emacs -Q example shows what may happen then:

(let* ((b1 (get-buffer-create "*b1*"))
       (b2 (get-buffer-create "*b2*"))
       (b3 (get-buffer-create "*b3*"))
       (action '(display-buffer-in-side-window
                 (dedicated . t)
                 (side . left)))
       (window (display-buffer b1 action)))
  (message "%s" (window-dedicated-p window))
  (sit-for 3)
  (display-buffer b2 action)
  (message "%s" (window-dedicated-p window))
  (sit-for 3)
  (display-buffer b3 t))

*b3* is shown in the side window although this is reserved for buffers
displayed via 'display-buffer-in-side-window'.

I checked in a fix.  Please try again.

Thanks for the report, martin

Message #14 received at 76911 <at> debbugs.gnu.org (full text, mbox):

From: Daniel Colascione <dancol <at> dancol.org>
To: martin rudalics <rudalics <at> gmx.at>
Cc: Eli Zaretskii <eliz <at> gnu.org>, 76911 <at> debbugs.gnu.org
Subject: Re: bug#76911: dedicated side windows lose their dedication upon reuse
Date: Sun, 16 Mar 2025 22:25:42 -0400

martin rudalics <rudalics <at> gmx.at> writes:

>>> Consider the code below:
>>>
>>>      (let ((b1 (get-buffer-create "*b1*"))
>>>            (b2 (get-buffer-create "*b2*"))
>>>            (action '(display-buffer-in-side-window
>>>                      (dedicated . t)
>>>                      (side . left))))
>>>        (display-buffer b1 action)
>>>        (sit-for 1)
>>>        (display-buffer b2 action))
>>>
>>> It puts b1 in a side window, then reuses that side window for b2.
>>> When we display b2, the window loses the strong dedication it had to b1
>>> despite the display action for b2 including a request for a
>>> dedicated window.
>>>
>>> I'd expect the window to be dedicated to b2 at the end of the
>>> above snippet.
>>
>> Martin, any comments?
>
> The following emacs -Q example shows what may happen then:
>
> (let* ((b1 (get-buffer-create "*b1*"))
>        (b2 (get-buffer-create "*b2*"))
>        (b3 (get-buffer-create "*b3*"))
>        (action '(display-buffer-in-side-window
>                  (dedicated . t)
>                  (side . left)))
>        (window (display-buffer b1 action)))
>   (message "%s" (window-dedicated-p window))
>   (sit-for 3)
>   (display-buffer b2 action)
>   (message "%s" (window-dedicated-p window))
>   (sit-for 3)
>   (display-buffer b3 t))
>
> *b3* is shown in the side window although this is reserved for buffers
> displayed via 'display-buffer-in-side-window'.
>
> I checked in a fix.  Please try again.
>
> Thanks for the report, martin

Thanks for the fix. The original bug is gone. I've noticed a few times
buffers being in side windows I didn't intend to have, but I haven't
been able to find a repro.  Probably an unrelated bug.  We should
probably visually style side windows differently by default.

Message #17 received at 76911 <at> debbugs.gnu.org (full text, mbox):

From: martin rudalics <rudalics <at> gmx.at>
To: Daniel Colascione <dancol <at> dancol.org>
Cc: Eli Zaretskii <eliz <at> gnu.org>, 76911 <at> debbugs.gnu.org
Subject: Re: bug#76911: dedicated side windows lose their dedication upon reuse
Date: Mon, 17 Mar 2025 09:58:01 +0100

> Thanks for the fix. The original bug is gone. I've noticed a few times
> buffers being in side windows I didn't intend to have, but I haven't
> been able to find a repro.  Probably an unrelated bug.

You could try without passing (dedicated . t) as alist element.  The
manual says

          The dedicated flag (*note Dedicated Windows::) has a slightly
          different meaning for side windows.  When a side window is
          created, that flag is set to the value ‘side’ to prevent
          ‘display-buffer’ to use the window in other action functions.
          Its value persists across invocations of ‘quit-window’,
          ‘kill-buffer’, ‘previous-buffer’ and ‘next-buffer’.

So a dedicated flag that does not equal 'side' might cause the problem
here.  If it persists, I can write some code for 'display-buffer' so we
can find the offender.

> We should
> probably visually style side windows differently by default.

We could try to paint the window dividers accordingly.

martin

Message #20 received at 76911 <at> debbugs.gnu.org (full text, mbox):

From: Daniel Colascione <dancol <at> dancol.org>
To: martin rudalics <rudalics <at> gmx.at>
Cc: Eli Zaretskii <eliz <at> gnu.org>, 76911 <at> debbugs.gnu.org
Subject: Re: bug#76911: dedicated side windows lose their dedication upon reuse
Date: Mon, 17 Mar 2025 17:46:07 -0400

On March 17, 2025 4:58:01 AM EDT, martin rudalics <rudalics <at> gmx.at> wrote:
>> Thanks for the fix. The original bug is gone. I've noticed a few times
>> buffers being in side windows I didn't intend to have, but I haven't
>> been able to find a repro.  Probably an unrelated bug.
>
>You could try without passing (dedicated . t) as alist element.  The
>manual says
>
>          The dedicated flag (*note Dedicated Windows::) has a slightly
>          different meaning for side windows.  When a side window is
>          created, that flag is set to the value ‘side’ to prevent
>          ‘display-buffer’ to use the window in other action functions.
>          Its value persists across invocations of ‘quit-window’,
>          ‘kill-buffer’, ‘previous-buffer’ and ‘next-buffer’.
>
>So a dedicated flag that does not equal 'side' might cause the problem
>here.  If it persists, I can write some code for 'display-buffer' so we
>can find the offender.

But I want the strong dedication semantics. Specifically, I'm looking for something like an omnipresent always available panel I can summon and dismiss any time with a simple keybinding. I also have facilities for cycling its size and extracting the omnipresent thing into a conventional window temporarily. I want not only strong dedication, but also no-other-window and no-delete-other-windows, which I set. Modulo bugs, it all seems to work.

For finding those bugs, maybe we need a history of recent window changes we can examine after something has gone wrong. Just today I got into a weird state in which I had two side windows with the same slot and have no idea how I got into that state or how I'd repro.

>
>> We should
>> probably visually style side windows differently by default.
>
>We could try to paint the window dividers accordingly.
>

Fringe arrows or something? Not sure what it should look like. I'm a programmer, not an artist. :-)

Message #23 received at 76911 <at> debbugs.gnu.org (full text, mbox):

From: martin rudalics <rudalics <at> gmx.at>
To: Daniel Colascione <dancol <at> dancol.org>
Cc: Eli Zaretskii <eliz <at> gnu.org>, 76911 <at> debbugs.gnu.org
Subject: Re: bug#76911: dedicated side windows lose their dedication upon reuse
Date: Tue, 18 Mar 2025 10:02:34 +0100

>> So a dedicated flag that does not equal 'side' might cause the problem
>> here.  If it persists, I can write some code for 'display-buffer' so we
>> can find the offender.
>
> But I want the strong dedication semantics. Specifically, I'm looking
> for something like an omnipresent always available panel I can summon
> and dismiss any time with a simple keybinding. I also have facilities
> for cycling its size and extracting the omnipresent thing into a
> conventional window temporarily. I want not only strong dedication,
> but also no-other-window and no-delete-other-windows, which I
> set. Modulo bugs, it all seems to work.

You might be asking a bit too much here: The side windows mechanism does
its own handling of dedicatedness which means that if you specify
'display-buffer-in-side-window' for a buffer-2 and the window at that
slot is strongly dedicated to buffer-1, buffer-2 will be still displayed
in that window.  There's no other way because I cannot display buffer-2
reasonably anywhere else.

BTW why do you set 'no-delete-other-windows' for a side window?  You
cannot avoid an error when incidentally doing C-x 5 1 in it I suppose.

> For finding those bugs, maybe we need a history of recent window
> changes we can examine after something has gone wrong. Just today I
> got into a weird state in which I had two side windows with the same
> slot and have no idea how I got into that state or how I'd repro.

As a matter of fact, this should never happen.  You could try running
Emacs with the patch below - it might help to detect such a situation.

martin

diff --git a/lisp/window.el b/lisp/window.el
index 1e8f4e323bc..dd7a463aa3b 100644
--- a/lisp/window.el
+++ b/lisp/window.el
@@ -7824,6 +7824,12 @@ window--display-buffer
         (dolist (parameter parameters)
           (set-window-parameter
            window (car parameter) (cdr parameter)))))
+    (with-current-buffer (get-buffer-create "*display-buffer-history*")
+      (goto-char (point-max))
+      (insert (format " window: %s" window)
+	      (format " [%s]" type)
+	      (format " alist: %s" alist)
+	      (format " parameters: %s\n" (window-parameters window))))
     window))

 (defun window--maybe-raise-frame (frame)
@@ -8220,6 +8226,9 @@ display-buffer
 	(split-window-keep-point t)
 	;; Handle the old form of the first argument.
 	(inhibit-same-window (and action (not (listp action)))))
+    (with-current-buffer (get-buffer-create "*display-buffer-history*")
+      (goto-char (point-max))
+      (insert (format "buffer: %s" buffer) (format " action: %s\n" action)))
     (unless (listp action) (setq action nil))
     (let* ((user-action
             (display-buffer-assq-regexp

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