bug#37683: Eww doesn't render Arabic script correctly

[Eli Zaretskii <eliz@gnu.org>]

> From: Nicholas Drozd <nicholasdrozd@gmail.com>
> Date: Wed, 9 Oct 2019 12:16:16 -0500
> 
> Here is the Persian Wikipedia page for Emacs:
> https://fa.wikipedia.org/wiki/%D8%A7%DB%8C%D9%85%DA%A9%D8%B3
> 
> When I open it in Eww, the cursive Perso-Arabic characters are not
> strung together properly. For example, the article mentions the
> "editor war", جنگ ویرایشگرها. In Eww, this is rendered as جنگ
> ویرایشگرها with wrong forms (initial vs middle vs final vs isolated).
> `D` runs `eww-toggle-paragraph-direction`, but that doesn't have any
> effect on the problem. I think this might be a cursive issue rather
> than a directionality issue, but I could be wrong.
> 
> (As I prepare to send this, I notice that Gmail won't render that
> phrase incorrectly, so the issue might not be communicated clearly
> here.)

Thank you for your report.

First, you cannot effectively report text-shaping (and in general,
rendering) problems by pasting the problematic text into an email
message, because most Emacs maintainers use Emacs to read email, so
any problems you report will be invisible to them, especially if you
show two similar segments of text that are supposed to be rendered
differently.  The only reliable way of reporting such rendering
problems is by including screenshots of the wrong and the correct
rendering (the latter from some other application or from a different
Emacs version, if that's applicable).

And second, you didn't provide any details about the Emacs version you
used and how it was built (i.e. the features it can support) -- this
information is collected by "M-x report-emacs-bug" when you invoke
it.  My first suspicion is that you are using Emacs 26 or older and/or
an outdated version of the libm17n library.  Emacs 27 uses HarfBuzz by
default (if available on the system), and I cannot imagine that
HarfBuzz doesn't render Farsi text correctly.  (If you send
screenshots, I can verify that on my system, as I don't read Farsi, so
I need an image of correct display to compare to.)