Subject: 27.1; Unicode noncharacters may change writing direction

From: [email-masked]
Date: Wed, 04 May 2022 09:06:37 +0200

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.

In GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.20, cairo version 1.16.0)
 of 2020-09-19 built on lgw01-amd64-021
Windowing system distributor 'The X.Org Foundation', version 11.0.12013000
System Description: Ubuntu 20.04.4 LTS

Recent messages:
Char: ﷐‎ (64976, #o176720, #xfdd0, file ...) point=4361 of 7190 (61%) column=23
Mark set [2 times]
Auto-saving...done

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