bug#42256: 27.0.50; composition

[Eli Zaretskii <eliz@gnu.org>]

> Date: Mon, 13 Jul 2020 06:37:52 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: rms@gnu.org, 42256@debbugs.gnu.org
> 
> > > That said, if someone wants to work on adding the character names to
> > > the GUI display as well, I won't object.
> > 
> > Ok, done on master in commit 46a0c115f0.
> 
> Please also update the Emacs manual, where it describes this display,
> because now the text there is outdated.

Actually, the results are inaccurate or even incorrect, at least in
some cases.  Here's one case where the results are wrong:

  emacs -Q
  C-h h
  C-u 411 M-g c
  C-u C-x =

You will see towards the end of the *help* buffer:

 Composed with the following character(s) "്" using this font:
   harfbuzz:-outline-Kartika-normal-normal-normal-serif-13-*-*-*-p-*-iso10646-1
 by these glyphs:
   [0 1 3384 337 12 0 12 9 0 nil]
 from these character(s):
   സ (#xd38) MALAYALAM LETTER SA
   ് (#xd4d) MALAYALAM SIGN VIRAMA
   ക (#xd15) MALAYALAM LETTER KA
   ാ (#xd3e) MALAYALAM VOWEL SIGN AA

The added list of characters seems to imply that 4 characters were
composed at buffer position 411.  But actually only the first 2 of
them were composed, as clearly see from the line starting with
"Composed with" above.

A similar problem happens at buffer position 413 of HELLO.  And at
position 872 you can see an even more stark example: instead of 2
characters, we show 8(!).

Can this please be fixed?