bug#28312: 25.2; Arabic script changes when scrolling

[Nick Helm <nick@tenpoint.co.nz>]

Eli Zaretskii <eliz@gnu.org> writes:

>> Observe the text on the line that starts with the word Arabic. As the
>> buffer scrolls, some of the characters in the Arabic representations of
>> the words "Arabic" and "Hello" change between at least two glyphs.
>
> Does the character also change its shape if you move cursor across
> that line, especially when the cursor moves across the character?

If I use the keyboard to move the cursor, no. The cursor changes shape
to reflect the width of the glyph, and the direction of cursor movement
correctly changes when over RTL text, as expected, but the chars
themselves do not change.

If I use the mouse to place the cursor in the line, yes. Glyphs near the
cursor change shape and stay changed until I release <mouse-1>.

>> Here's a quick screen cap to show what I mean:
>> https://ln.sync.com/dl/22d37f320/fwujc85a-dhb63yfw-cv5vhzj5-5qsis2h8
>
> I cannot watch this movie with the software I have installed.

Gah, sorry. Try this:
https://ln.sync.com/dl/c0466f180/ju7j9ka3-ebq4uzfz-2wp4ehnu-3szvp5jd

> Arabic needs shaping support from the underlying font back-end, so
> (unless this will be reproduced on GNU/Linux) it sounds like something
> is wrong with either the back-end Emacs uses on Darwin, or its version
> you have installed, or with the support code in Emacs which is needed
> for displaying complex RTL scripts such as Arabic.

When I open HELLO using LibreOffice, which also supports RTL on mac, the
problem doesn't seem to occur using Geeza Pro or Arial Unicode MS. Not
sure if that is relevant.