bug#28312: 25.2; Arabic script changes when scrolling

[Eli Zaretskii]

> Date: Fri, 01 Sep 2017 16:36:13 +0900
> From: YAMAMOTO Mitsuharu <mituharu@math.s.chiba-u.ac.jp>
> Cc: Nick Helm <nick@tenpoint.co.nz>,
>       28312@debbugs.gnu.org
> 
> 1. emacs -Q &
> 2. M-x describe-language-environment RET Arabic RET
> 3. Press and release the mouse button around the Arabic sample text.
>    (say, "Arabic" to the left)
> 
> The appearance of the sample text changes if I press the button, and
> it changes back again when I release it.

Thanks, I see this, too.  And it's a very old problem, I see it in
Emacs 24.3 already.

Also, the mouse is not necessary, I can see the problem if I scroll
the *Help* window created by "C-h L" either via the scroll bar or even
with "C-u 1 C-v".

I'm guessing that somehow we don't redraw enough characters to give
the font shaper a chance to shape them correctly.  CC'ing Handa-san,
in the hope that he could have some insights on this.