bug#41005: problem with rendering Persian text in Emacs 27

[Amin Bandali <bandali@gnu.org>]

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Eli Zaretskii <eliz@gnu.org> writes:

[...]
>
> Yes, that would be most useful.
>
> If the result still points into some valid Emacs use pattern, please
> describe in more detail a small part of the Wikipedia page that needs
> to be copy/pasted, and the precise place where the result of pasting
> is rendered incorrectly.  Please keep in mind that for people who
> don't read Persian it is hard to find those issues in vast amounts of
> text, so any measure that makes this significantly easier will allow
> debugging and finding solutions to be much more efficient.
>
> Thanks.
>

Having done some digging, I've found at least one reliable way to
reproduce the issue with -Q:

1. launch Emacs using `emacs -Q';
2. switch to *scratch* if not there already;
3. do M-x column-number-mode RET;
4. paste the following Persian text into *scratch*:

   ویکی‌پدیا دانشنامه‌ای اینترنتی با بیش از ۲۸۰ زبان با محتوای آزاد است که با همکاری افراد داوطلب نوشته می‌شود و هر کس که به اینترنت دسترسی داشته باشد می‌تواند مقاله‌های آن را ویرایش کند.

   It is from the first sentence of the "دربارهٔ ویکی‌پدیا" (About
   Wikipedia) section on <https://fa.wikipedia.org>.

   The text should appear garbled.  However, if you omit step 3, the
   pasted text should appear just fine.  It appears that if Persian text
   is pasted before column-number-mode is enabled, even if the paste is
   subsequently undone before enabling column-number-mode, the issue
   does *not* surface.

The above works for pasting into a message-mode buffer as well.  I have
not tried to find a recipe specific to gnus-article-mode yet.

Hope this helps.

P.S. if I did not mention earlier, all of this is on a Debian Buster
GNU/Linux system.

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