GNU bug report logs - #28312
25.2; Arabic script changes when scrolling

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Message #23 received at 28312 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: YAMAMOTO Mitsuharu <mituharu <at> math.s.chiba-u.ac.jp>,
 Kenichi Handa <handa <at> gnu.org>
Cc: 28312 <at> debbugs.gnu.org, nick <at> tenpoint.co.nz
Subject: Re: bug#28312: 25.2; Arabic script changes when scrolling
Date: Fri, 01 Sep 2017 11:35:38 +0300

> Date: Fri, 01 Sep 2017 16:36:13 +0900
> From: YAMAMOTO Mitsuharu <mituharu <at> math.s.chiba-u.ac.jp>
> Cc: Nick Helm <nick <at> tenpoint.co.nz>,
> 	28312 <at> 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.

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