bug#28312: 25.2; Arabic script changes when scrolling

[YAMAMOTO Mitsuharu]

>>>>> On Fri, 01 Sep 2017 09:45:08 +0300, Eli Zaretskii <eliz@gnu.org> said:

>> With bidi enabled, Arabic text changes unexpectedly when scrolling
>> or moving point with the mouse.
>> 
>> Recipe: Emacs -Q C-h h ;visit HELLO file ;use the mouse to scroll
>> slowly
>> 
>> 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.

> I don't see this here, but I'm not on Darwin.  Can anyone reproduce
> this on GNU/Linux?

I could reproduce a similar effect also on Darwin, but a different
font backend driver (xft with libotf 0.9.3, m17n lib/db 1.7.0).

1. emacs -Q &
2. M-x describe-language-environment RET Arabic RET
3. Press and release the mouse button around the Arabic sample text.
   (say, "Arabic" to the left)

The appearance of the sample text changes if I press the button, and
it changes back again when I release it.

The font used for displaying Arabic characters is "Arial Unicode MS".

                                     YAMAMOTO Mitsuharu
                                mituharu@math.s.chiba-u.ac.jp