bug#23292: 24.5; Combining characters do not reliably combine

[Honore Doktorr]

Combining characters often do not render combined in emacs 24.5.
For example, the combining short solidus overlay (0x337, o̷) appears
following the character it's meant to combine with---although they are
both highlighted together and so forth.

cf. https://i.imgsafe.org/8369941.jpeg for a visual reference.

This seems similar to bug 17261, but both characters are from the same
font, DejaVu Sans Mono.

describe-char for the combined (but separately rendered) o̷:
---------------------------------------------------------------------------
             position: 14 of 70 (19%), column: 12
            character: o (displayed as o) (codepoint 111, #o157, #x6f)
    preferred charset: ascii (ASCII (ISO646 IRV))
code point in charset: 0x6F
               script: latin
               syntax: w     which means: word
             category: .:Base, L:Left-to-right (strong), a:ASCII,
l:Latin, r:Roman
             to input: type "C-x 8 RET HEX-CODEPOINT" or "C-x 8 RET NAME"
          buffer code: #x6F
            file code: #x6F (encoded by coding system utf-8)
              display: composed to form "o̷" (see below)

Composed with the following character(s) "̷" using this font:
  xft:-PfEd-DejaVu Sans Mono-normal-normal-normal-*-13-*-*-*-m-0-iso10646-1
by these glyphs:
  [0 1 111 82 8 1 7 7 0 nil]
  [0 1 823 703 8 0 8 8 1 nil]

Character code properties: customize what to show
  name: LATIN SMALL LETTER O
  general-category: Ll (Letter, Lowercase)
  decomposition: (111) ('o')
---------------------------------------------------------------------------

describe-char for the combining short solidus overlay by itself:
---------------------------------------------------------------------------
            position: 619 of 1008 (61%), column: 42
            character: ̷ (displayed as ̷) (codepoint 823, #o1467, #x337)
    preferred charset: unicode-bmp (Unicode Basic Multilingual Plane
(U+0000..U+FFFF))
code point in charset: 0x0337
               script: latin
               syntax: w     which means: word
             category: ^:Combining
             to input: type "C-x 8 RET HEX-CODEPOINT" or "C-x 8 RET NAME"
          buffer code: #xCC #xB7
            file code: #xCC #xB7 (encoded by coding system utf-8)
              display: composed to form "̷" (see below)

Composed by the rule:
    (TAB ?̷ TAB)
The component character(s) are displayed by these fonts (glyph codes):
 ̷: xft:-PfEd-DejaVu Sans
Mono-normal-normal-normal-*-13-*-*-*-m-0-iso10646-1 (#x2BF)
See the variable `reference-point-alist' for the meaning of the rule.

Character code properties: customize what to show
  name: COMBINING SHORT SOLIDUS OVERLAY
  old-name: NON-SPACING SHORT SLASH OVERLAY
  general-category: Mn (Mark, Nonspacing)
  decomposition: (823) ('̷')

There are text properties here:
  composition          [Show]
---------------------------------------------------------------------------




In GNU Emacs 24.5.1 (x86_64-redhat-linux-gnu, GTK+ Version 3.18.7)
 of 2016-02-03 on buildhw-05.phx2.fedoraproject.org
Windowing system distributor `Fedora Project', version 11.0.11803000
System Description:    Fedora release 23 (Twenty Three)

Configured using:
 `configure --build=x86_64-redhat-linux-gnu
 --host=x86_64-redhat-linux-gnu --program-prefix=
 --disable-dependency-tracking --prefix=/usr --exec-prefix=/usr
 --bindir=/usr/bin --sbindir=/usr/sbin --sysconfdir=/etc
 --datadir=/usr/share --includedir=/usr/include --libdir=/usr/lib64
 --libexecdir=/usr/libexec --localstatedir=/var
 --sharedstatedir=/var/lib --mandir=/usr/share/man
 --infodir=/usr/share/info --with-dbus --with-gif --with-jpeg --with-png
 --with-rsvg --with-tiff --with-xft --with-xpm --with-x-toolkit=gtk3
 --with-gpm=no build_alias=x86_64-redhat-linux-gnu
 host_alias=x86_64-redhat-linux-gnu 'CFLAGS=-DMAIL_USE_LOCKF -O2 -g
 -pipe -Wall -Werror=format-security -Wp,-D_FORTIFY_SOURCE=2
 -fexceptions -fstack-protector-strong --param=ssp-buffer-size=4
 -grecord-gcc-switches -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1
 -m64 -mtune=generic' LDFLAGS=-Wl,-z,relro'

Important settings:
  value of $LANG: en_US.utf8
  value of $XMODIFIERS: @im=none
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  show-paren-mode: t
  delete-selection-mode: t
  display-time-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  temp-buffer-resize-mode: t
  column-number-mode: t
  line-number-mode: t
  transient-mark-mode: t

Recent messages:
Mark set [2 times]
Type "q" to restore previous buffer, M-x scroll-up to scroll help.
byte-code: End of buffer
<mouse-6> is undefined
byte-code: Beginning of buffer [4 times]
Quit
<<< Type SPC or RET to bury the buffer list >>>

byte-code: Beginning of buffer [2 times]


Load-path shadows:
/home/denis/.emacs.d/elpa/js2-mode-20141118/js2-mode hides
/home/denis/lib/site-lisp/…aside/js2-mode
~/lib/site-lisp/markdown-mode hides
/usr/share/emacs/site-lisp/goodies/markdown-mode
~/lib/site-lisp/css-mode hides /usr/share/emacs/24.5/lisp/textmodes/css-mode
/usr/share/emacs/site-lisp/gnus-bonus/nnir hides
/usr/share/emacs/24.5/lisp/gnus/nnir
/usr/share/emacs/site-lisp/gnus-bonus/nnnil hides
/usr/share/emacs/24.5/lisp/gnus/nnnil
/usr/share/emacs/site-lisp/gnus-bonus/spam-stat hides
/usr/share/emacs/24.5/lisp/gnus/spam-stat
~/lib/site-lisp/longlines hides /usr/share/emacs/24.5/lisp/obsolete/longlines

Features:
(shadow sort gnus-util mail-extr emacsbug message idna format-spec
rfc822 mml mml-sec mm-decode mm-bodies mm-encode mail-parse rfc2231
mailabbrev gmm-utils mailheader sendmail rfc2047 rfc2045 ietf-drums
mm-util mail-prsvr mail-utils pp mule-util ebuff-menu wid-edit
descr-text help-mode package epg-config server follow-mouse desktop
frameset saveplace paren cl-macs cl gv apropos derived markdown-mode
cl-loaddefs cl-lib thingatpt noutline outline easymenu advice help-fns
mustache-mode delsel grep compile comint ansi-color ring cus-start
cus-load time 50magit emacs-goodies-loaddefs easy-mmode time-date
tooltip electric uniquify ediff-hook vc-hooks lisp-float-type mwheel
x-win x-dnd tool-bar dnd fontset image regexp-opt fringe tabulated-list
newcomment lisp-mode prog-mode register page menu-bar rfn-eshadow timer
select scroll-bar mouse jit-lock font-lock syntax facemenu font-core
frame cham georgian utf-8-lang misc-lang vietnamese tibetan thai
tai-viet lao korean japanese hebrew greek romanian slovak czech european
ethiopic indian cyrillic chinese case-table epa-hook jka-cmpr-hook help
simple abbrev minibuffer nadvice loaddefs button faces cus-face macroexp
files text-properties overlay sha1 md5 base64 format env code-pages mule
custom widget hashtable-print-readable backquote make-network-process
dbusbind gfilenotify dynamic-setting system-font-setting
font-render-setting move-toolbar gtk x-toolkit x multi-tty emacs)

Memory information:
((conses 16 129620 19595)
 (symbols 48 32900 0)
 (miscs 40 1221 515)
 (strings 32 37972 9706)
 (string-bytes 1 1003542)
 (vectors 16 19979)
 (vector-slots 8 1305892 204196)
 (floats 8 78 475)
 (intervals 56 445 16)
 (buffers 960 16)
 (heap 1024 49802 1271))