On Sat, Oct 2, 2021 at 2:18 PM Eli Zaretskii <eliz@gnu.org> wrote:

What font displays them individually? You should be able to tell that
if you type "C-u C-x =" on one of these characters.

This is the default font that `emacs -Q` chooses. Using "C-u C-x =" I see,

ftcrhb:-PfEd-DejaVu Sans-normal-normal-normal-*-13-*-*-*-*-0-iso10646-1 (#x56F)

So would it be correct to say that using a proper font solves the
problem?

This is almost correct except that the font (NotoNastaliqUrdu) that causes problems with emacs

works fine in libreoffice and both use harfbuzz.

I don't think the problem is with compositions, because in the 2
examples you described above, there are no character compositions.

Moreover, our pattern for asking HarfBuzz to shape Arabic text is
this:

"[\u0600-\u074F\u200C\u200D]+"

which includes all of the characters, including U+06C1 which you say
causes problems.

You could try setting current-iso639-language to the symbol 'ur'
(without the quotes), that should tell HarfBuzz to shape the text as
appropriate for Urdu. But I think the real problem is with the font,
not with shaping.

Sorry, my lack of understanding of terminology got in the way here. I thought composition referred to

joining characters together.

I did try setting `current-iso639-language` to the symbol `ur`. As you expected it didn't make a difference.