bug#41645: 27.0.91; Combining Grapheme Joiner (#x34f) gui artifacts

[Eli Zaretskii]

> Date: Mon, 01 Jun 2020 21:37:38 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: dfussner@googlemail.com, 41645@debbugs.gnu.org
> 
>   Composed with the following character(s) "͏" using this font:
>     harfbuzz:-outline-Code2000-normal-normal-normal-*-16-*-*-*-p-*-iso8859-1
>   by these glyphs:
>     [0 1 100 71 10 0 9 11 0 nil]
>     [0 1 847 3 6 0 1 0 1 [0 0 0]]
> 
> Is what you see on your system similar?  If so, which glyph is
> zero-width, and why does it cause trouble on your system, but not on
> mine?

And another question: in the cases where you see artifacts, does the
call to font-shape-gstring inside compose-gstring-for-graphic return
nil or non-nil?  IOW, is the shaper+font producing the composed glyph,
or is that the fallback Lisp code in compose-gstring-for-graphic,
because font-shape-gstring returns nil?  Perhaps that's the reason for
the differences between what you see and what I see.