bug#41506: 28.0.50; RTL problem

[Eli Zaretskii <eliz@gnu.org>]

> From: Pip Cet <pipcet@gmail.com>
> Date: Tue, 2 Jun 2020 10:17:55 +0000
> Cc: 41506@debbugs.gnu.org
> 
> > It's a bug, but one that's very tricky to fix, AFAIR.  If you insert
> > or delete a single character, the display becomes correct.
> 
> The attached patch seems to avoid the problem, but I'm sure I'm
> missing something.

This condition was added 11 years ago, when I just started integrating
bidi.c with Emacs.  The commit log message and the comment both say I
had some real problem on my hands that this condition fixed.  However,
I have now thrown several use cases on the patched code, and could see
no problem.

So I guess whatever issues I had back then were meanwhile solved "by
other means", and you should install this patch.  If there is indeed
some subtlety here, it will present itself sooner or later (like, in
another 11 years).

> I suspect what might have been meant is that narrowing an LTR
> paragraph to a line containing STRONG_R text shouldn't result in RTL
> display, but it does...

No, this works as designed: the Emacs display engine always behaves as
if there's nothing before beginning of the narrowed region.

Thanks.