bug#35721: 27.0.50; Strange Arabic shaping behavior

[Basil L. Contovounesios]

I see the following on the master, harfbuzz, and emacs-26 branches
(precise versions follow my signature), but I'm not sure how much of
this is expected or due to e.g. my font.

0. emacs -Q
1. C-x 8 RET 0634 RET

The "tail" of the sheen is truncated by the fringe:

2. C-a C-u C-x =

--8<---------------cut here---------------start------------->8---
             position: 146 of 146 (99%), column: 0
            character: ش‎ (displayed as ش‎) (codepoint 1588, #o3064, #x634)
              charset: unicode (Unicode (ISO10646))
code point in charset: 0x0634
               script: arabic
               syntax: w        which means: word
             category: .:Base, R:Right-to-left (strong), b:Arabic
             to input: type "C-x 8 RET 634" or "C-x 8 RET ARABIC LETTER SHEEN"
          buffer code: #xD8 #xB4
            file code: #xD8 #xB4 (encoded by coding system utf-8-unix)
              display: by this font (glyph code)
    xft:-PfEd-DejaVu Sans Mono-normal-normal-normal-*-19-*-*-*-m-0-iso10646-1 
(#x46A)

Character code properties: customize what to show
  name: ARABIC LETTER SHEEN
  general-category: Lo (Letter, Other)
  decomposition: (1588) ('ش')

There are text properties here:
  fontified            nil
--8<---------------cut here---------------end--------------->8---

3. SPC

The "tail" of the sheen becomes visible, but falls outside of the box
cursor:

4. C-x 8 RET 0643 RET

The kaf is correctly shaped in its initial form:

5. C-SPC

The kaf changes to its isolated form:

6. C-g C-a C-k
7. C-u C-\ arabic RET
8. a ; RET

The sheen is correctly shaped in its initial form and the kaf is
truncated by the fringe:

9. a ; RET

The first sheen unexpectedly changes to its isolated form:

I occasionally see this happen even without typing anything, as if by a
timer, but I'm not sure how to reproduce it.  I think, without being
100% certain, that it's only happened while using the 'arabic' input
method.

10. a

The first sheen reverts to its initial form:

Any insights?  Thanks,

-- 
Basil

In GNU Emacs 27.0.50 (build 40, x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll 
bars)
 of 2019-05-13 built on thunk
Repository revision: a1e5cce99b75c1bd50995b7b4d81423b1296fa60
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12003000
System Description: Debian GNU/Linux buster/sid

Configured using:
 'configure 'CC=ccache gcc' 'CFLAGS=-O2 -march=native' --config-cache
 --prefix=/home/blc/.local --with-mailutils --with-x-toolkit=lucid
 --with-modules --with-file-notification=yes --with-x'

Configured features:
XAW3D XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS
GLIB NOTIFY INOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT
LIBOTF XFT ZLIB TOOLKIT_SCROLL_BARS LUCID X11 XDBE XIM MODULES THREADS
LIBSYSTEMD JSON PDUMPER LCMS2 GMP

Important settings:
  value of $LANG: en_IE.UTF-8
  locale-coding-system: utf-8-unix


In GNU Emacs 27.0.50 (build 2, x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll 
bars)
 of 2019-05-13 built on thunk
Repository revision: 5d7dafacf4afc888511649f6fc24c28210cd0dfc
Repository branch: harfbuzz
Windowing system distributor 'The X.Org Foundation', version 11.0.12003000
System Description: Debian GNU/Linux buster/sid

Configured using:
 'configure 'CC=ccache gcc' 'CFLAGS=-O0 -g3 -ggdb -gdwarf-4'
 --config-cache --prefix=/home/blc/.local --program-suffix=-harfbuzz
 --enable-checking=yes,glyphs --enable-check-lisp-object-type
 --with-mailutils --with-x-toolkit=lucid --with-modules
 --with-file-notification=yes --with-x'

Configured features:
XAW3D XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS
GLIB NOTIFY INOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ
M17N_FLT LIBOTF XFT ZLIB TOOLKIT_SCROLL_BARS LUCID X11 XDBE XIM MODULES
THREADS LIBSYSTEMD JSON PDUMPER LCMS2 GMP


In GNU Emacs 26.2.50 (build 2, x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll 
bars)
 of 2019-04-30 built on thunk
Repository revision: c26d452ae15a74f0eeec53ba529eebaa95eb5489
Windowing system distributor 'The X.Org Foundation', version 11.0.12003000
System Description:     Debian GNU/Linux buster/sid

Configured using:
 'configure 'CC=ccache gcc' 'CFLAGS=-O0 -g3 -ggdb -gdwarf-4'
 --config-cache --prefix=/home/blc/.local --program-suffix=26
 --enable-checking=yes,glyphs --enable-check-lisp-object-type
 --with-mailutils --with-x-toolkit=lucid --with-modules
 --with-file-notification=yes --with-x'

Configured features:
XAW3D XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS
GLIB NOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT
ZLIB TOOLKIT_SCROLL_BARS LUCID X11 XDBE XIM MODULES THREADS LIBSYSTEMD
LCMS2