bug#44784: Combining accent is not combining

[Juri Linkov]

> Btw, the report by Emacs 27's "C-u C-x =", viz.:
>
>   Composed with the following character(s) "́" using this font:
>     xfthb:-PfEd-DejaVu Sans 
> Mono-normal-normal-normal-*-22-*-*-*-m-0-iso10646-1
>   by these glyphs:
>     [0 1 1071 878 13 0 13 16 0 nil]
>     [0 1 769 649 13 4 11 17 -13 [0 0 0]]
>
> explicitly says that character composition _did_ happen here, it's
> just the display that is broken.  And I guess that moving point to
> these characters will show a "fat" cursor that includes both the base
> character and the accent, once again showing that Emacs did compose
> the accent with the preceding character.

Actually, separate cursor is shown on a character and separate on its
combining character, so it seems they are not composed.

>> 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?
>
> 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:

Yes, it displays correctly with other fonts, e.g. Noto Sans.
It would be a pity to switch to another font.  So this bug report
could be closed if nothing can be done in Emacs.