bug#69972: 29.1; Unexpected behavior when scrolling images

bug#69972: 29.1; Unexpected behavior when scrolling images

[Joseph Turner via Bug reports for GNU Emacs, the Swiss army knife of text editors]

When the image at point is larger than the current window and there is
no content after the image, interactively scrolling down (with the
scroll-up command) unexpectedly scrolls past the image to blankness.

Test this by evaluating the following snippet then interactively running
`scroll-up' repeatedly:

(with-current-buffer (get-buffer-create "*test-scroll-image*")
  (erase-buffer)
  (insert-image (create-image "splash.png" nil nil
    ;; Scale the image more if it doesn't take up the whole window.
    :scale 5))
  (goto-char (point-max))
  (pop-to-buffer (current-buffer)))

Even more unexpectedly, when point is before the image, running
`scroll-up' repeatedly eventually scrolls back to the top of the image:

(with-current-buffer (get-buffer-create "*test-scroll-image*")
  (erase-buffer)
  (insert-image (create-image "splash.png" nil nil :scale 5))
  (goto-char (point-min))
  (pop-to-buffer (current-buffer)))

Non-interactively, scroll-up scrolls the page down to the very bottom of
the image.  IMO this is the expected behavior:

(with-current-buffer (get-buffer-create "*test-scroll-image*")
  (erase-buffer)
  (insert-image (create-image "splash.png" nil nil :scale 5))
  (pop-to-buffer (current-buffer))
  (scroll-up 999))

Thank you for your help!!

Joseph

Tested on emacs -Q:

In GNU Emacs 29.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.37,
cairo version 1.16.0)
Windowing system distributor 'The X.Org Foundation', version 11.0.12101007
System Description: Debian GNU/Linux 12 (bookworm)

Configured using:
 'configure
 CONFIG_SHELL=/gnu/store/v9p25q9l5nnaixkhpap5rnymmwbhf9rp-bash-minimal-5.1.16/bin/bash
 SHELL=/gnu/store/v9p25q9l5nnaixkhpap5rnymmwbhf9rp-bash-minimal-5.1.16/bin/bash
 --prefix=/gnu/store/f4gxh8n3libmywq8d5jvxxkyqblgilxy-emacs-29.1
 --enable-fast-install --with-cairo --with-modules
 --with-native-compilation=aot --disable-build-details'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES
NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG RSVG SECCOMP SOUND SQLITE3
THREADS TIFF TOOLKIT_SCROLL_BARS TREE_SITTER X11 XDBE XIM XINPUT2 XPM
GTK3 ZLIB

bug#69972: 29.1; Unexpected behavior when scrolling images

[Eli Zaretskii]

> Date: Sun, 24 Mar 2024 02:07:16 -0700
> From:  Joseph Turner via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> When the image at point is larger than the current window and there is
> no content after the image, interactively scrolling down (with the
> scroll-up command) unexpectedly scrolls past the image to blankness.

I cannot reproduce this, I think.

> Test this by evaluating the following snippet then interactively running
> `scroll-up' repeatedly:
> 
> (with-current-buffer (get-buffer-create "*test-scroll-image*")
>   (erase-buffer)
>   (insert-image (create-image "splash.png" nil nil
>     ;; Scale the image more if it doesn't take up the whole window.
>     :scale 5))
>   (goto-char (point-max))
>   (pop-to-buffer (current-buffer)))

This recipe doesn't include the call to scroll-up, so I'm unsure how
you did that and what you saw.  When I try "M-: (scroll-up) RET" or
"M-x scroll-up RET", I get several scrolls by window-size, and then
"End of buffer" error when I hit the end of the buffer.  If this is
unexpected, please tell why.

> Even more unexpectedly, when point is before the image, running
> `scroll-up' repeatedly eventually scrolls back to the top of the image:
> 
> (with-current-buffer (get-buffer-create "*test-scroll-image*")
>   (erase-buffer)
>   (insert-image (create-image "splash.png" nil nil :scale 5))
>   (goto-char (point-min))
>   (pop-to-buffer (current-buffer)))

I do see this, but why is that a problem?  You supposed to use
scroll-up-command instead, which handles these marginal cases much
better.  scroll-up itself is not smart enough to avoid the perceived
"scroll back to top", which is actually caused by the fact that we
zero out window-vscroll (which is how we handle scrolling past large
images).

bug#69972: 29.1; Unexpected behavior when scrolling images

[Joseph Turner via Bug reports for GNU Emacs, the Swiss army knife of text editors]

Eli Zaretskii <eliz@gnu.org> writes:

>> Date: Sun, 24 Mar 2024 02:07:16 -0700
>> From:  Joseph Turner via "Bug reports for GNU Emacs,
>>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
>>
>> When the image at point is larger than the current window and there is
>> no content after the image, interactively scrolling down (with the
>> scroll-up command) unexpectedly scrolls past the image to blankness.
>
> I cannot reproduce this, I think.
>
>> Test this by evaluating the following snippet then interactively running
>> `scroll-up' repeatedly:
>>
>> (with-current-buffer (get-buffer-create "*test-scroll-image*")
>>   (erase-buffer)
>>   (insert-image (create-image "splash.png" nil nil
>>     ;; Scale the image more if it doesn't take up the whole window.
>>     :scale 5))
>>   (goto-char (point-max))
>>   (pop-to-buffer (current-buffer)))
>
> This recipe doesn't include the call to scroll-up, so I'm unsure how
> you did that and what you saw.  When I try "M-: (scroll-up) RET" or
> "M-x scroll-up RET", I get several scrolls by window-size, and then
> "End of buffer" error when I hit the end of the buffer.  If this is
> unexpected, please tell why.

On my machine, I don't get "End of buffer" error.  I tested three
different ways in the following snippet

(with-current-buffer (get-buffer-create "*test-scroll-image*")
  (erase-buffer)
  (insert-image (create-image "splash.png" nil nil
    ;; Scale the image more if it doesn't take up the whole window.
    :scale 5))
  (goto-char (point-max))
  (pixel-scroll-precision-mode -1)  ; Ensure mouse wheel scroll up and down works
  (pop-to-buffer (current-buffer)))

1. "M-: (scroll-up) RET" (repeatedly)
2. C-v (repeatedly)
3. <wheel-down> (repeatedly)

With all three methods, at first Emacs gradually scrolls the image, but
then when I reach the bottom of the image, the image disappears entirely
as Emacs scrolls past it all at once.

>> Even more unexpectedly, when point is before the image, running
>> `scroll-up' repeatedly eventually scrolls back to the top of the image:
>>
>> (with-current-buffer (get-buffer-create "*test-scroll-image*")
>>   (erase-buffer)
>>   (insert-image (create-image "splash.png" nil nil :scale 5))
>>   (goto-char (point-min))
>>   (pop-to-buffer (current-buffer)))
>
> I do see this, but why is that a problem?  You supposed to use
> scroll-up-command instead, which handles these marginal cases much
> better.  scroll-up itself is not smart enough to avoid the perceived
> "scroll back to top", which is actually caused by the fact that we
> zero out window-vscroll (which is how we handle scrolling past large
> images).

You're right.  This is not a problem in practice.  When point is before
the image, both C-v and <wheel-down> produce the same behavior as above.

Joseph

bug#69972: 29.1; Unexpected behavior when scrolling images

[Joseph Turner via Bug reports for GNU Emacs, the Swiss army knife of text editors]

Joseph Turner <joseph@ushin.org> writes:

> Eli Zaretskii <eliz@gnu.org> writes:
>
>>> Date: Sun, 24 Mar 2024 02:07:16 -0700
>>> From:  Joseph Turner via "Bug reports for GNU Emacs,
>>>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
>>>
>>> When the image at point is larger than the current window and there is
>>> no content after the image, interactively scrolling down (with the
>>> scroll-up command) unexpectedly scrolls past the image to blankness.
>>
>> I cannot reproduce this, I think.
>>
>>> Test this by evaluating the following snippet then interactively running
>>> `scroll-up' repeatedly:
>>>
>>> (with-current-buffer (get-buffer-create "*test-scroll-image*")
>>>   (erase-buffer)
>>>   (insert-image (create-image "splash.png" nil nil
>>>     ;; Scale the image more if it doesn't take up the whole window.
>>>     :scale 5))
>>>   (goto-char (point-max))
>>>   (pop-to-buffer (current-buffer)))
>>
>> This recipe doesn't include the call to scroll-up, so I'm unsure how
>> you did that and what you saw.  When I try "M-: (scroll-up) RET" or
>> "M-x scroll-up RET", I get several scrolls by window-size, and then
>> "End of buffer" error when I hit the end of the buffer.  If this is
>> unexpected, please tell why.
>
> On my machine, I don't get "End of buffer" error.

Correction - I *do* get "End of buffer" error, but only after fully
scrolling past the image (which IMO is unexpected).

> I tested three different ways in the following snippet
>
> (with-current-buffer (get-buffer-create "*test-scroll-image*")
>   (erase-buffer)
>   (insert-image (create-image "splash.png" nil nil
>     ;; Scale the image more if it doesn't take up the whole window.
>     :scale 5))
>   (goto-char (point-max))
>   (pixel-scroll-precision-mode -1)  ; Ensure mouse wheel scroll up and down works
>   (pop-to-buffer (current-buffer)))
>
> 1. "M-: (scroll-up) RET" (repeatedly)
> 2. C-v (repeatedly)
> 3. <wheel-down> (repeatedly)
>
> With all three methods, at first Emacs gradually scrolls the image, but
> then when I reach the bottom of the image, the image disappears entirely
> as Emacs scrolls past it all at once.
>
>>> Even more unexpectedly, when point is before the image, running
>>> `scroll-up' repeatedly eventually scrolls back to the top of the image:
>>>
>>> (with-current-buffer (get-buffer-create "*test-scroll-image*")
>>>   (erase-buffer)
>>>   (insert-image (create-image "splash.png" nil nil :scale 5))
>>>   (goto-char (point-min))
>>>   (pop-to-buffer (current-buffer)))
>>
>> I do see this, but why is that a problem?  You supposed to use
>> scroll-up-command instead, which handles these marginal cases much
>> better.  scroll-up itself is not smart enough to avoid the perceived
>> "scroll back to top", which is actually caused by the fact that we
>> zero out window-vscroll (which is how we handle scrolling past large
>> images).
>
> You're right.  This is not a problem in practice.  When point is before
> the image, both C-v and <wheel-down> produce the same behavior as above.
>
> Joseph

bug#69972: 29.1; Unexpected behavior when scrolling images

[Eli Zaretskii]

> From: Joseph Turner <joseph@ushin.org>
> Cc: 69972@debbugs.gnu.org
> Date: Sun, 24 Mar 2024 18:06:44 -0700
> 
> (with-current-buffer (get-buffer-create "*test-scroll-image*")
>   (erase-buffer)
>   (insert-image (create-image "splash.png" nil nil
>     ;; Scale the image more if it doesn't take up the whole window.
>     :scale 5))
>   (goto-char (point-max))
>   (pixel-scroll-precision-mode -1)  ; Ensure mouse wheel scroll up and down works
>   (pop-to-buffer (current-buffer)))
> 
> 1. "M-: (scroll-up) RET" (repeatedly)
> 2. C-v (repeatedly)
> 3. <wheel-down> (repeatedly)
> 
> With all three methods, at first Emacs gradually scrolls the image, but
> then when I reach the bottom of the image, the image disappears entirely
> as Emacs scrolls past it all at once.

Why is that a problem?  The default Emacs scrolling commands don't
guarantee smooth constant-rate scrolling, they guarantee only that you
get to see all the portions of the image, without missing any.  So
when you get to the end of the image, there's a heuristic that decides
whether we can scroll past the image, and that is what you see.

I see no bug here.

bug#69972: 29.1; Unexpected behavior when scrolling images

[Joseph Turner via Bug reports for GNU Emacs, the Swiss army knife of text editors]

Eli Zaretskii <eliz@gnu.org> writes:

>> From: Joseph Turner <joseph@ushin.org>
>> Cc: 69972@debbugs.gnu.org
>> Date: Sun, 24 Mar 2024 18:06:44 -0700
>>
>> 1. "M-: (scroll-up) RET" (repeatedly)
>> 2. C-v (repeatedly)
>> 3. <wheel-down> (repeatedly)
>>
>> With all three methods, at first Emacs gradually scrolls the image, but
>> then when I reach the bottom of the image, the image disappears entirely
>> as Emacs scrolls past it all at once.
>
> Why is that a problem?  The default Emacs scrolling commands don't
> guarantee smooth constant-rate scrolling, they guarantee only that you
> get to see all the portions of the image, without missing any.  So
> when you get to the end of the image, there's a heuristic that decides
> whether we can scroll past the image, and that is what you see.
>
> I see no bug here.

With pixel-scroll-precision-mode enabled and at least one character after
the image, I am able to smoothly scroll around the image.

(with-current-buffer (get-buffer-create "*test-scroll-image*")
  (erase-buffer)
  (insert-image (create-image "splash.png" nil nil :scale 10))
  (insert " ") ;; There must be some text after the image
  (goto-char (point-min))
  (pixel-scroll-precision-mode 1)
  (pop-to-buffer (current-buffer)))

However, if the window is scrolled to the right, then it is impossible
to scroll down:

(with-current-buffer (get-buffer-create "*test-scroll-image*")
  (erase-buffer)
  (insert-image (create-image "splash.png" nil nil :scale 10))
  (insert-char 8203)
  (goto-char (point-min))
  (pixel-scroll-precision-mode 1)
  (pop-to-buffer (current-buffer))
  (set-window-hscroll (get-buffer-window) 1)) ;; Image scrolled to right

In this buffer, if I attempt to scroll down with the mouse wheel, I
immediately get "End of buffer" error and the image does not move.

Thank you!

Joseph

bug#69972: 29.1; Unexpected behavior when scrolling images

[Eli Zaretskii]

> From: Joseph Turner <joseph@ushin.org>
> Cc: 69972@debbugs.gnu.org
> Date: Mon, 25 Mar 2024 12:43:39 -0700
> 
> >> With all three methods, at first Emacs gradually scrolls the image, but
> >> then when I reach the bottom of the image, the image disappears entirely
> >> as Emacs scrolls past it all at once.
> >
> > Why is that a problem?  The default Emacs scrolling commands don't
> > guarantee smooth constant-rate scrolling, they guarantee only that you
> > get to see all the portions of the image, without missing any.  So
> > when you get to the end of the image, there's a heuristic that decides
> > whether we can scroll past the image, and that is what you see.
> >
> > I see no bug here.
> 
> With pixel-scroll-precision-mode enabled and at least one character after
> the image, I am able to smoothly scroll around the image.

pixel-scroll-precision-mode indeed attempts to make the scrolling much
more smooth than the default scrolling in these cases.

> However, if the window is scrolled to the right, then it is impossible
> to scroll down:
> 
> (with-current-buffer (get-buffer-create "*test-scroll-image*")
>   (erase-buffer)
>   (insert-image (create-image "splash.png" nil nil :scale 10))
>   (insert-char 8203)
>   (goto-char (point-min))
>   (pixel-scroll-precision-mode 1)
>   (pop-to-buffer (current-buffer))
>   (set-window-hscroll (get-buffer-window) 1)) ;; Image scrolled to right
> 
> In this buffer, if I attempt to scroll down with the mouse wheel, I
> immediately get "End of buffer" error and the image does not move.

Then maybe Po Lu (CC'ed) could look into this and see if
pixel-scroll-precision-mode could do better in this case.

bug#69972: 29.1; Unexpected behavior when scrolling images

[Po Lu via Bug reports for GNU Emacs, the Swiss army knife of text editors]

Eli Zaretskii <eliz@gnu.org> writes:

> Then maybe Po Lu (CC'ed) could look into this and see if
> pixel-scroll-precision-mode could do better in this case.

Thanks.  I think a solution to the OP's problem was proposed in a
previous bug report and we're still awaiting feedback, though at the
moment I don't have its bug number at hand.