bug#35024: 26.1; unicode combining characters displayed incorrectly in emacs-26.1

[Basil L. Contovounesios]

qamats.png
Description: PNG image

Mayer Goldberg <gmayer@little-lisper.org> writes:

> Hello:
>
> ?ש = \u05e9 = "HEBREW LETTER SHIN"
> ?ָ = \u05b8 = "HEBREW POINT QAMATS"
>
> i.e., "\u05e9\u05b8", should display with the qamats right under the shin,
> which is the case under emacs 25 and earlier, but fails in emacs 26: The
> qamats is printed below but to the side, and not right under the letter
> shin. This happens regardless of the font used, and with professional
> OTF fonts that are designed to handle combining characters correctly,
> and which are handled correctly by emacs 25 and earlier. This problem is
> not unique to Hebrew, but is true for any RTL language I've checked
> inclinding Arabic and Farsi.

FWIW, I don't see the behaviour you describe from 'emacs -Q' using a
checkout of the emacs-26 branch (what will be Emacs 26.2):

0. emacs -Q
1. M-: (insert "\u05e9\u05b8") RET
2. C-b C-u C-x =

This is what what-cursor-position reports:

--8<---------------cut here---------------start------------->8---
             position: 146 of 147 (99%), column: 0
            character: ש‎ (displayed as ש‎) (codepoint 1513, #o2751, #x5e9)
              charset: unicode (Unicode (ISO10646))
code point in charset: 0x05E9
               script: hebrew
               syntax: w        which means: word
             category: .:Base, R:Right-to-left (strong)
             to input: type "C-x 8 RET 5e9" or "C-x 8 RET HEBREW LETTER SHIN"
          buffer code: #xD7 #xA9
            file code: #xD7 #xA9 (encoded by coding system utf-8-unix)
              display: composed to form "שָ" (see below)

Composed with the following character(s) "ָ" using this font:
  xft:-PfEd-Linux Libertine Display 
O-normal-normal-normal-*-15-*-*-*-*-0-iso10646-1
by these glyphs:
  [0 1 1513 1005 9 0 9 9 0 nil]
  [0 1 1464 967 0 -7 -4 -1 4 [-8 0 0]]

Character code properties: customize what to show
  name: HEBREW LETTER SHIN
  general-category: Lo (Letter, Other)
  decomposition: (1513) ('ש')

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

And attached is a screenshot of how it looks on my screen (scaled with
repeated calls to text-scale-adjust).

-- 
Basil

In GNU Emacs 26.1.92 (build 20, x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll 
bars)
 of 2019-03-21 built on thunk
Repository revision: 0924b27bca40d219e34529144ea04a581428f1f7
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 -pipe'
 --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

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