bug#28312: 25.2; Arabic script changes when scrolling

[Eli Zaretskii <eliz@gnu.org>]

> From: Nick Helm <nick@tenpoint.co.nz>
> CC: "mituharu@math.s.chiba-u.ac.jp" <mituharu@math.s.chiba-u.ac.jp>,
> 	"handa@gnu.org" <handa@gnu.org>, "28312@debbugs.gnu.org"
> 	<28312@debbugs.gnu.org>
> Date: Sun, 9 Jun 2019 23:56:22 +0000
> 
> > Would people please try the patch and see if Emacs is still reasonably
> > responsive in buffers with Arabic text, especially when marking or
> > extending the region under transient-mark-mode?  If I get enough
> > positive feedback, I will install this.  TIA.
> 
> I built yesterday's master and tested with and without the patch.
> 
> With the patch applied, I can confirm that the Arabic text in HELLO no
> longer changes when scrolling, either with the keyboard or mouse.

Thanks for testing.

> However, I noticed that it still seems to change when extending the
> region over Arabic text. That is, place point on the "c" of Arabic,
> C-SPC, and C-f multiple times to extend the region and notice the
> graphemes change. The mouse click and release method described up-thread
> also shows the same thing. 

This is a separate problem, one whose solution would basically need
redesigning how we handle faces in redisplay.  The problem is that we
currently process buffer text for display in chunks that have the same
face, so if the region starts or ends in the middle of an Arabic word,
we submit this word to the shaping engine in 2 chunks, thus breaking
character composition.  This is very basic in the display engine's
design, so don't hold your breath on that one.

> Next, I opened a file written entirely in Arabic (~3000 words). Without
> the patch, scrolling works as expected in all cases I tried. With the
> patch applied, scrolling also works as expected unless pixel-scroll-mode
> is active and scrolling is done with the mouse. With that mode active,
> mouse scrolling introduces lag that takes 3–4 seconds to recover after
> each scroll action. I would describe the lag as significant. I did not
> notice the text changing when scrolling or clicking in this file.

I'm working on a less expensive solution, stay tuned.