bug#16840: 24.3.50; Jerky motion and up/down asymmetry scrolling images in Eww

[N. Jackson]

Hi Eli,

At 13:04 -0400 on Friday 2015-12-25, Eli Zaretskii wrote:

>> From: Lars Ingebrigtsen <larsi@gnus.org>
>> Cc: 16840@debbugs.gnu.org,  nljlistbox2@gmail.com
>> Date: Fri, 25 Dec 2015 17:54:26 +0100
>> 
>> > Can you show a URL and describe the gestures to reproduce the
>> > problem?
>> 
>> I don't have any handy, and they are probably dependent on the
>> screen size, the font size and the window sizes. But these glitches
>> usually show up after a while if you use eww on web sites that have
>> many images, like newspaper web sites.
>
> How about if you tell me the next time you see one of those?

Testing this afternoon, here's a recipe that shows the problem for me
(on Emacs 24):

1. emacs -Q [1]

2. M-x eww

3. Enter http://www.fsf.org/campaigns/ at the prompt [2]

4. M-x split-window-below (this is to make the images taller than the
window)

5. Scroll down the page with C-n, down to the heading "Secure Boot vs
Restricted Boot" (about 51 lines)

6. C-n to the blank line above the image

7. C-n eight times (this scrolls the image at one would expect, one
"line" at a time)

8. C-n again
   
[1] Emacs 24.5. Default font is coming up as
-xos4-Terminus-normal-normal-normal-*-12-*-*-*-c-60-iso10646-1.

[2] Accessed 2015-12-25 18:13 UTC.

The `C-n's in Step 7 scroll the image as one would expect. At the
second last of those `C-n's, the heading "Secure Boot vs Restricted
Boot" scrolls off the top of the window. At the next `C-n' the blank
line below the image is at the top of the window.

At the next `C-n' (Step 8) which I understand is supposed to move
point to the line below the image [Although it would be awesome if it
could continue scrolling the image one "line" at a time.], the
scrolling "glitches" and point moves _up_ again to the blank line
above the image.

I will install Emacs 25 now and report back whether I still see the
same behaviour on this system.

Regards,
Neil.