bug#56008: 29.0.50; image-mode buffer scrolled down automatically

bug#56008: 29.0.50; image-mode buffer scrolled down automatically

[Visuwesh]

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image-mode buffers are scrolled down without user intervention sometimes
when another buffer content is updated.  I can reliably reproduce this
using eww.  To demonstrate,

    1. emacs -Q
    2. Visit attached image file.
    3. s o ;; to make the window show only some part of the image
    4. M->
    5. C-x 3 ;; split the frame into two windows
    6. M-s M-w search for something here RET
    7. C-x o ;; switch to the image-mode buffer
    8. Observe image-mode buffer moving to the beginning of image when
       the eww buffer gets updated with the webpage content.

(7,8) You have to be quick enough to switch back to the image-mode
buffer as otherwise, this behaviour is not seen.

By (2), I mean to say that the image should be large enough such that
only a part of the image is shown in the window, unlike what is seen
when you visit the file initially.

This does not happen only with eww but in other scenarios as well.  But
I am unable to reproduce this behaviour in those scenarios as they are a
lot more involved.


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In GNU Emacs 29.0.50 (build 17, x86_64-pc-linux-gnu, X toolkit, Xaw scroll bars)
 of 2022-06-15 built on astatine
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bug#56008: 29.0.50; image-mode buffer scrolled down automatically

[Lars Ingebrigtsen]

Visuwesh <visuweshm@gmail.com> writes:

> image-mode buffers are scrolled down without user intervention sometimes
> when another buffer content is updated.  I can reliably reproduce this
> using eww.  To demonstrate,
>
>     1. emacs -Q
>     2. Visit attached image file.
>     3. s o ;; to make the window show only some part of the image
>     4. M->
>     5. C-x 3 ;; split the frame into two windows
>     6. M-s M-w search for something here RET
>     7. C-x o ;; switch to the image-mode buffer
>     8. Observe image-mode buffer moving to the beginning of image when
>        the eww buffer gets updated with the webpage content.

I can reproduce this -- what a strange error.

Hm...  anybody got any ideas how to debug this?  Something is apparently
resetting the window-start in the wrong window?  Or...  something else?

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

bug#56008: 29.0.50; image-mode buffer scrolled down automatically

[Lars Ingebrigtsen]

Lars Ingebrigtsen <larsi@gnus.org> writes:

> Hm...  anybody got any ideas how to debug this?  Something is apparently
> resetting the window-start in the wrong window?  Or...  something else?

Hm!  If I say `M-:' afterwards, the window position in the Image buffer
window flips back to where it was supposed to be.  So this seems like
it's a redisplay bug and not a programming error somewhere (I was
wondering if some shr code was setting window-start in the wrong window
or something like that).

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

bug#56008: 29.0.50; image-mode buffer scrolled down automatically

[Eli Zaretskii]

> Cc: 56008@debbugs.gnu.org
> From: Lars Ingebrigtsen <larsi@gnus.org>
> Date: Thu, 16 Jun 2022 14:04:54 +0200
> 
> >     1. emacs -Q
> >     2. Visit attached image file.
> >     3. s o ;; to make the window show only some part of the image
> >     4. M->
> >     5. C-x 3 ;; split the frame into two windows
> >     6. M-s M-w search for something here RET
> >     7. C-x o ;; switch to the image-mode buffer
> >     8. Observe image-mode buffer moving to the beginning of image when
> >        the eww buffer gets updated with the webpage content.
> 
> I can reproduce this -- what a strange error.

Indeed.

> Hm...  anybody got any ideas how to debug this?  Something is apparently
> resetting the window-start in the wrong window?  Or...  something else?

Something else, because window-start stays put at 1.  image-mode
implements "M->" by setting vscroll and hscroll, it doesn't change
window-start.  Moreover, if you type "M-x" after the recipe, the image
gets redisplayed as expected after "M->".  Which means that the
window's vscroll didn't change, either.

bug#56008: 29.0.50; image-mode buffer scrolled down automatically

[Visuwesh]

[வியாழன் ஜூன் 16, 2022 14:19] Lars Ingebrigtsen wrote:

> Lars Ingebrigtsen <larsi@gnus.org> writes:
>
>> Hm...  anybody got any ideas how to debug this?  Something is apparently
>> resetting the window-start in the wrong window?  Or...  something else?
>
> Hm!  If I say `M-:' afterwards, the window position in the Image buffer
> window flips back to where it was supposed to be.

Oh, I knew that but that totally slipped my mind.  Sorry about that.

>  So this seems like it's a redisplay bug and not a programming error
> somewhere (I was wondering if some shr code was setting window-start
> in the wrong window or something like that).

Yes, it is not specific to shr/eww.  That's what I alluded to by saying,

    This does not happen only with eww but in other scenarios as well.  But
    I am unable to reproduce this behaviour in those scenarios as they are a
    lot more involved.

bug#56008: 29.0.50; image-mode buffer scrolled down automatically

[Eli Zaretskii]

> Cc: 56008@debbugs.gnu.org
> From: Visuwesh <visuweshm@gmail.com>
> Date: Thu, 16 Jun 2022 18:49:49 +0530
> 
> >  So this seems like it's a redisplay bug and not a programming error
> > somewhere (I was wondering if some shr code was setting window-start
> > in the wrong window or something like that).
> 
> Yes, it is not specific to shr/eww.  That's what I alluded to by saying,
> 
>     This does not happen only with eww but in other scenarios as well.  But
>     I am unable to reproduce this behaviour in those scenarios as they are a
>     lot more involved.

No, this _is_ specific to eww and shr.  Well, in a way: shr.el does
something which directly causes this.  Stay tuned.

bug#56008: 29.0.50; image-mode buffer scrolled down automatically

[Lars Ingebrigtsen]

Eli Zaretskii <eliz@gnu.org> writes:

> No, this _is_ specific to eww and shr.  Well, in a way: shr.el does
> something which directly causes this.  Stay tuned.

I wondered whether it had something to do with shr-string-pixel-width
and friends (which does play games with windows), but stubbing those out
still led to the same problems, apparently.

(shr-string-pixel-width should be replaced with string-pixel-width (sort
of), though.)

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

bug#56008: 29.0.50; image-mode buffer scrolled down automatically

[Visuwesh]

[வியாழன் ஜூன் 16, 2022 16:05] Lars Ingebrigtsen wrote:

> Eli Zaretskii <eliz@gnu.org> writes:
>
>> No, this _is_ specific to eww and shr.  Well, in a way: shr.el does
>> something which directly causes this.  Stay tuned.
>

FWIW, I first saw this when using ement.el which inserts text in buffers
whenever it receives a matrix message.  The bug report I filed in ement.el bug
reporter can be found here: https://github.com/alphapapa/ement.el/issues/58.  
Maybe it is of some value, but I could never get to the root of the
problem.  And I heard another user seeing something similar as well,

    just me or sometimes pdf-view-mode from pdf-tools will automatically
    reset buffer position to initial position (like it just scrolls up the
    buffer). The scenario in which it happens is when I have a frame with
    two windows vertically splited, window 1 is for org-mode, and window 2
    is for pdf-view-mode, and current focus is on window 1.

    seems to be triggered after `Creating LaTeX preview... done.` by
    org-latex-preview and org-fragtog, and when saving the file.

> I wondered whether it had something to do with shr-string-pixel-width
> and friends (which does play games with windows), but stubbing those out
> still led to the same problems, apparently.
>
> (shr-string-pixel-width should be replaced with string-pixel-width (sort
> of), though.)

When I tried to dig around in ement.el's source code, I thought it was
`save-window-excursion' but that turned out not to be the case AFAIU.

bug#56008: 29.0.50; image-mode buffer scrolled down automatically

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: Visuwesh <visuweshm@gmail.com>,  56008@debbugs.gnu.org
> Date: Thu, 16 Jun 2022 16:05:20 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > No, this _is_ specific to eww and shr.  Well, in a way: shr.el does
> > something which directly causes this.  Stay tuned.
> 
> I wondered whether it had something to do with shr-string-pixel-width
> and friends (which does play games with windows), but stubbing those out
> still led to the same problems, apparently.
> 
> (shr-string-pixel-width should be replaced with string-pixel-width (sort
> of), though.)

It's shr-pixel-column, which calls set-window-buffer.

bug#56008: 29.0.50; image-mode buffer scrolled down automatically

[Eli Zaretskii]

> From: Visuwesh <visuweshm@gmail.com>
> Cc: Eli Zaretskii <eliz@gnu.org>,  56008@debbugs.gnu.org
> Date: Thu, 16 Jun 2022 19:44:13 +0530
> 
> When I tried to dig around in ement.el's source code, I thought it was
> `save-window-excursion' but that turned out not to be the case AFAIU.

What do you mean by "it was `save-window-excursion'"? what is "it" in
this case?  And how did you see that this is not the case?

bug#56008: 29.0.50; image-mode buffer scrolled down automatically

[Visuwesh]

[வியாழன் ஜூன் 16, 2022 18:49] Eli Zaretskii wrote:

>> From: Visuwesh <visuweshm@gmail.com>
>> Cc: Eli Zaretskii <eliz@gnu.org>,  56008@debbugs.gnu.org
>> Date: Thu, 16 Jun 2022 19:44:13 +0530
>> 
>> When I tried to dig around in ement.el's source code, I thought it was
>> `save-window-excursion' but that turned out not to be the case AFAIU.
>
> What do you mean by "it was `save-window-excursion'"? what is "it" in
> this case?

I thought the source of the random, unwarranted scrolling could be
`save-window-excursion' since it did come with the following warning,

    BEWARE: Most uses of this macro introduce bugs.
    E.g. it should not be used to try and prevent some code from opening
    a new window, since that window may sometimes appear in another frame,
    in which case ‘save-window-excursion’ cannot help.

>  And how did you see that this is not the case?

Sorry, but it is close to a year since I looked at the source code.  All
I remember is being annoyed by the bug despite removing the
`save-window-excursion' bit willy-nilly.

[ This was all in ement.el's source code if I wasn't clear, BTW.  ]

bug#56008: 29.0.50; image-mode buffer scrolled down automatically

[Eli Zaretskii]

> Cc: 56008@debbugs.gnu.org, visuweshm@gmail.com
> Date: Thu, 16 Jun 2022 18:48:26 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> 
> > From: Lars Ingebrigtsen <larsi@gnus.org>
> > Cc: Visuwesh <visuweshm@gmail.com>,  56008@debbugs.gnu.org
> > Date: Thu, 16 Jun 2022 16:05:20 +0200
> > 
> > Eli Zaretskii <eliz@gnu.org> writes:
> > 
> > > No, this _is_ specific to eww and shr.  Well, in a way: shr.el does
> > > something which directly causes this.  Stay tuned.
> > 
> > I wondered whether it had something to do with shr-string-pixel-width
> > and friends (which does play games with windows), but stubbing those out
> > still led to the same problems, apparently.
> > 
> > (shr-string-pixel-width should be replaced with string-pixel-width (sort
> > of), though.)
> 
> It's shr-pixel-column, which calls set-window-buffer.

And in addition, shr-insert-document was resetting the hscroll of the
window it just happened to use for its rendering job.

This bug should be fixed now on the master branch.

bug#56008: 29.0.50; image-mode buffer scrolled down automatically

[Eli Zaretskii]

> From: Visuwesh <visuweshm@gmail.com>
> Cc: larsi@gnus.org,  56008@debbugs.gnu.org
> Date: Thu, 16 Jun 2022 22:25:05 +0530
> 
> [வியாழன் ஜூன் 16, 2022 18:49] Eli Zaretskii wrote:
> 
> > What do you mean by "it was `save-window-excursion'"? what is "it" in
> > this case?
> 
> I thought the source of the random, unwarranted scrolling could be
> `save-window-excursion' since it did come with the following warning,
> 
>     BEWARE: Most uses of this macro introduce bugs.
>     E.g. it should not be used to try and prevent some code from opening
>     a new window, since that window may sometimes appear in another frame,
>     in which case ‘save-window-excursion’ cannot help.

We don't create new windows in this case, so that warning is not
relevant.

But save-window-excursion is indeed part of the puzzle, see below.

> >  And how did you see that this is not the case?
> 
> Sorry, but it is close to a year since I looked at the source code.  All
> I remember is being annoyed by the bug despite removing the
> `save-window-excursion' bit willy-nilly.
> 
> [ This was all in ement.el's source code if I wasn't clear, BTW.  ]

I don't know what ement.el does and whether this is related, but
here's what was happening in this case:

  . we invoke eww to fetch some URL
  . before it finishes to fetch the URL, we switch to another window
  . eww finishes fetching the URL and calls shr.el to render it
  . shr.el uses the selected window for rendering, and as part of
    that, it calls set-window-buffer to temporarily make the buffer in
    which it renders be the buffer of the selected window
  . set-window-buffer resets the window's hscroll and vscroll values
  . shr.el calls set-window-buffer inside save-window-excursion, but
    save-window-excursion didn't preserve the value of vscroll, and in
    addition shr.el called set-window-hscroll in one place that wasn't
    inside save-window-excursion
  . so both vscroll and hscroll of the window showing the image ended
    up with zero hscroll and vscroll
  . since image-mode overrides the usual definition of M-> with a
    command that works by setting the window's hscroll and vscroll,
    resetting these two makes the (mistaken) impression that the
    window was scrolled
  . the reason M-x and M-: appeared to "fix" the problem is that
    image-mode defines a window-configuration-change-hook that forces
    the display back to what it was -- this looked to us as if the
    problem was caused by some redisplay bug, something that took me
    some time to realize its being completely wrong

bug#56008: 29.0.50; image-mode buffer scrolled down automatically

[Visuwesh]

[வியாழன் ஜூன் 16, 2022 19:58] Eli Zaretskii wrote:

>> Cc: 56008@debbugs.gnu.org, visuweshm@gmail.com
>> Date: Thu, 16 Jun 2022 18:48:26 +0300
>> From: Eli Zaretskii <eliz@gnu.org>
>> 
>> > From: Lars Ingebrigtsen <larsi@gnus.org>
>> > Cc: Visuwesh <visuweshm@gmail.com>,  56008@debbugs.gnu.org
>> > Date: Thu, 16 Jun 2022 16:05:20 +0200
>> > 
>> > Eli Zaretskii <eliz@gnu.org> writes:
>> > 
>> > > No, this _is_ specific to eww and shr.  Well, in a way: shr.el does
>> > > something which directly causes this.  Stay tuned.
>> > 
>> > I wondered whether it had something to do with shr-string-pixel-width
>> > and friends (which does play games with windows), but stubbing those out
>> > still led to the same problems, apparently.
>> > 
>> > (shr-string-pixel-width should be replaced with string-pixel-width (sort
>> > of), though.)
>> 
>> It's shr-pixel-column, which calls set-window-buffer.
>
> And in addition, shr-insert-document was resetting the hscroll of the
> window it just happened to use for its rendering job.
>
> This bug should be fixed now on the master branch.

Can confirm that it is fixed, thanks for the quick fix!

bug#56008: 29.0.50; image-mode buffer scrolled down automatically

[Visuwesh]

[வியாழன் ஜூன் 16, 2022 20:11] Eli Zaretskii wrote:

>> From: Visuwesh <visuweshm@gmail.com>
>> Cc: larsi@gnus.org,  56008@debbugs.gnu.org
>> Date: Thu, 16 Jun 2022 22:25:05 +0530
>> 
>> [வியாழன் ஜூன் 16, 2022 18:49] Eli Zaretskii wrote:
>> 
>> > What do you mean by "it was `save-window-excursion'"? what is "it" in
>> > this case?
>> 
>> I thought the source of the random, unwarranted scrolling could be
>> `save-window-excursion' since it did come with the following warning,
>> 
>>     BEWARE: Most uses of this macro introduce bugs.
>>     E.g. it should not be used to try and prevent some code from opening
>>     a new window, since that window may sometimes appear in another frame,
>>     in which case ‘save-window-excursion’ cannot help.
>
> We don't create new windows in this case, so that warning is not
> relevant.
>
> But save-window-excursion is indeed part of the puzzle, see below.
>
>> >  And how did you see that this is not the case?
>> 
>> Sorry, but it is close to a year since I looked at the source code.  All
>> I remember is being annoyed by the bug despite removing the
>> `save-window-excursion' bit willy-nilly.
>> 
>> [ This was all in ement.el's source code if I wasn't clear, BTW.  ]
>
> I don't know what ement.el does and whether this is related, but
> here's what was happening in this case:
>
>   . we invoke eww to fetch some URL
>   . before it finishes to fetch the URL, we switch to another window
>   . eww finishes fetching the URL and calls shr.el to render it
>   . shr.el uses the selected window for rendering, and as part of
>     that, it calls set-window-buffer to temporarily make the buffer in
>     which it renders be the buffer of the selected window
>   . set-window-buffer resets the window's hscroll and vscroll values
>   . shr.el calls set-window-buffer inside save-window-excursion, but
>     save-window-excursion didn't preserve the value of vscroll, and in
>     addition shr.el called set-window-hscroll in one place that wasn't
>     inside save-window-excursion
>   . so both vscroll and hscroll of the window showing the image ended
>     up with zero hscroll and vscroll
>   . since image-mode overrides the usual definition of M-> with a
>     command that works by setting the window's hscroll and vscroll,
>     resetting these two makes the (mistaken) impression that the
>     window was scrolled
>   . the reason M-x and M-: appeared to "fix" the problem is that
>     image-mode defines a window-configuration-change-hook that forces
>     the display back to what it was -- this looked to us as if the
>     problem was caused by some redisplay bug, something that took me
>     some time to realize its being completely wrong

Thanks for the explanation, Eli!  If I still the remember the ement.el
code, I think your commit _should_ fix the problem.  But if it doesn't,
at least now I have a better idea of where to look.  Thanks, again.

bug#56008: 29.0.50; image-mode buffer scrolled down automatically

[Eli Zaretskii]

> From: Visuwesh <visuweshm@gmail.com>
> Cc: larsi@gnus.org,  56008@debbugs.gnu.org
> Date: Thu, 16 Jun 2022 22:50:12 +0530
> 
> > This bug should be fixed now on the master branch.
> 
> Can confirm that it is fixed, thanks for the quick fix!

Thanks for testing; closing.