bug#56695: A new and old font problem with Arabic

[Eli Zaretskii <eliz@gnu.org>]

> Date: Fri, 22 Jul 2022 10:11:16 +0200 (CEST)
> From: "fr_ml@t-online.de" <fr_ml@t-online.de>
> 
> 1. I can't change the Arabic characters to the font of DejaVu Sans
> Mono (or others). This wasn't a problem before.

What do you mean by "cannot change"?  What happens if you try?  Please
show the exact sequence of commands or Lisp code you used, and any
error messages or incorrect effects you see when you do that.

> 2. The old one: As you can see the glyphs aren't correct, the '-'
> after س is not correctly placed. It's in the place of the glyph of
> س. Sometimes I have the opposite problem: gaps between the
> characters. It depends usually of 'text-scale-adjust', if I change
> the scale the problem can disappear.
> 
> (Of course I started Emacs with the '-Q' option.)
> 
> Attachment: To see the difference: a screenshot in Word/LibreOffice Writer:
> In LibreOffice:
> 1. The font is correct (also for the Arabic characters)
> 2. The '-' is at the correct place
> 
> Can Harfbuzz be a problem? In the older emacs version (maybe 3 or 4 years ago?) I never had these
> problems.

I don't know, because critical information is missing:

  . on what OS is that?
  . what is the version of HarfBuzz you have installed?
  . what is the text that is not displayed correctly? (please provide
    it as plain text that can be copied and examined, not as an image)
  . what does Emacs say when you go to the characters involved in this
    display and type "C-u C-x ="?

In general DejaVu Sans is known as a problematic font for complex text
shaping (which is required for Arabic), so my advice is to use some
other font as the default.

Thanks.