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

[Eli Zaretskii]

> From: Pip Cet <pipcet@gmail.com>
> Date: Mon, 1 Jun 2020 15:50:16 +0000
> Cc: David Fussner <dfussner@googlemail.com>, 41645@debbugs.gnu.org
> 
> > I cannot reproduce this on my system, FWIW.  Very strange.
> 
> I can reproduce this, but it depends on which font it uses.

I have only 2 fonts that cover that codepoint, and both produce
correct display.  When I try a font that doesn't support it, I see a
box with hex code, which, of course, causes no problems.

When you do reproduce the problem, do the characters around CGJ use
the same font as the CGJ itself, or do they use a different font?  If
the latter, the composition won't work, and maybe that is when the
problem happens?

> Is it possible it's bug 41454? I'm attaching that patch in case it
> does anything and people feel like testing it.

I'm guessing you already tested, and it doesn't fix the problem for
you (when you use the fonts which do reproduce the problem)?

I think that bug is unlikely to be the culprit: symptoms such as those
described here usually mean that Emacs becomes confused about the
metrics of the character's glyph on display.  So to find the root
cause, one should step through the code which draws the character and
then the code which draws the cursor on that character, and see what's
going on there with the metrics.