bug#55256: Writing direction

[Eli Zaretskii]

> Date: Wed, 04 May 2022 09:20:05 +0200
> From: frederik.fouvry--- via "Bug reports for GNU Emacs,
>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
> 
> 
> I forgot to add:
> 
> The Unicode noncharacters should not cause a change in writing
> direction, since they are not Arabic characters, but a set of characters
> for internal use only (no exchange between different parties).

That is not entirely true, because Unicode assigns default Bidi Class
properties to some unassigned codepoints, and Emacs obeys that.  So an
unassigned codepoint (which is AFAIU what "noncharacter" stands for in
your terminology) for which Unicode says that its Bidi Class should
be, for example, AL, _will_ cause change of text directionality.

If you use those unassigned codepoints for private use, you will have
to override the default properties by manually modifying the relevant
Emacs char-tables at run time.