bug#28312: 25.2; Arabic script changes when scrolling

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 10 Jun 2019 20:27:02 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: 28312@debbugs.gnu.org
> 
> > Next, I opened a file written entirely in Arabic (~3000 words). Without
> > the patch, scrolling works as expected in all cases I tried. With the
> > patch applied, scrolling also works as expected unless pixel-scroll-mode
> > is active and scrolling is done with the mouse. With that mode active,
> > mouse scrolling introduces lag that takes 3–4 seconds to recover after
> > each scroll action. I would describe the lag as significant. I did not
> > notice the text changing when scrolling or clicking in this file.
> 
> I'm working on a less expensive solution, stay tuned.

I installed a solution that should be much less expensive, please take
a look.  If you still see lag in scrolling, try enabling
garbage-collection-messages to see whether GC is the culprit; if it
is, then raise gc-cons-threshold while testing, to exclude this
factor's influence.

Thanks.