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

[Eli Zaretskii]

> From: Lars Ingebrigtsen <larsi@gnus.org>
> Cc: nljlistbox2@gmail.com,  16840@debbugs.gnu.org
> Date: Thu, 26 Sep 2019 20:34:59 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > This is all as expected, so there's no bug here.
> 
> Well, you may expect it, but this bug report is about how apparently a
> lot of other people find it surprising.

I don't understand why.

Your code generates an image whose height is smaller than the window
height, and in that case Emacs scrolls the entire image off the window
in one go, because all of the image was already visible once.  If I
decrease the frame height so that the image becomes taller than the
window, the image is scrolled partially until you had a chance to see
all of it, then the rest is scrolled away in one large step.  This is
exactly how the code in simple.el is designed and implemented, so I
see no bug here.

Once again, the principle is to make sure the user sees all of the
image before the image is scrolled off the viewport.