bug#44784: Combining accent is not combining

[Eli Zaretskii]

> From: Juri Linkov <juri@linkov.net>
> Date: Sat, 21 Nov 2020 21:17:09 +0200
> 
> 0. emacs -Q
> 1. insert to the *scratch* buffer:
> 
> ä́  і́  Я́ 
> 
> In 26.3 everything is correct, and COMBINING ACUTE ACCENT
> is combined above its previous character:
> 
> 
> But in 27.1 COMBINING ACUTE ACCENT is not combined with its previous 
> character:

On my system, this displays the same in both Emacs 26 and 27, the
latter using HarfBuzz.

> It seems in 26.3 it's correct because it uses XFT, but the problem in 27.1
> is with HarfBuzz .  Maybe HarfBuzz doesn't support combining accents on
> some characters?  It fails with the scripts above, but still has no problems
> with plain Latin characters.

That cannot be the reason.  Please try with other fonts, for
starters.  DejaVu Sans Mono is known to have problems with combining
accents, see this discussion:

  https://lists.freedesktop.org/archives/harfbuzz/2019-August/007422.html