bug#44521: 27.1; ivy.el breaks harfbuzz Arabic shaping

[Thamer Mahmoud <thamer.mahmoud@gmail.com>]

When using ivy.el, some Arabic letters become disconnected. This bug
doesn't affect Emacs 26.3 or any build "--without-harfbuzz".

1. Eval:

(progn
  (load-library "~/.emacs.d/elpa/ivy-0.13.0/ivy-overlay.el")
  (load-library "~/.emacs.d/elpa/ivy-0.13.0/colir.el")
  (load-library "~/.emacs.d/elpa/ivy-0.13.0/ivy.el")
  (ivy-mode t)
  (run-with-timer .5 nil 'set-input-method "arabic")
  (run-with-timer .5 nil 'insert "السلام عليكم")
  (execute-extended-command nil))

2. While still in the minibuffer, type any Arabic letter. Note the
   AIN(ع) in "عليكم" changes shape from initial to final.

This bug breaks shaping in most buffers, especially after using
swiper.el (ivy-based isearch).


libharfbuzz0b 2.6.7-1 from debian testing
In GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.23, cairo version 1.16.0)
 of 2020-11-08, modified by Debian built on x86-ubc-01
Windowing system distributor 'The X.Org Foundation', version 11.0.12008000
System Description: Debian GNU/Linux bullseye/sid

Configured features:
XPM JPEG TIFF GIF PNG RSVG CAIRO SOUND GPM DBUS GSETTINGS GLIB NOTIFY
INOTIFY ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ M17N_FLT LIBOTF
ZLIB TOOLKIT_SCROLL_BARS GTK3 X11 XDBE XIM MODULES THREADS LIBSYSTEMD
JSON PDUMPER LCMS2 GMP

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

-- 
Best,
Thamer