bug#55256: 27.1; Unicode noncharacters may change writing direction

[Eli Zaretskii]

> Date: Wed, 04 May 2022 09:06:37 +0200
> From: frederik.fouvry--- via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> 
> M-x set-input-method RET
> ucs RET
> jufdd0$1 RET
> 
> If you type C-a and then step through the characters with the right
> arrow, the direction is reversed. It seems like the entry of $ followed
> by something else is triggering it, but that may just be an
> impression/side effect.
> 
> I suspect that the reason is that most Unicode noncharacters
> (https://www.unicode.org/faq/private_use.html#nonchar1) are in an Arabic
> block (https://www.unicode.org/faq/private_use.html#nonchar4b) and that
> the cause is an incorrect generalisation of the properties of the
> characters in this block.

Correct.  We failed to be in sync with the Unicode Standard in this
regard.  Should be fixed now on the master branch.

Thanks.