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

[Eli Zaretskii <eliz@gnu.org>]

> From: Pip Cet <pipcet@gmail.com>
> Cc: dfussner@googlemail.com,  41645@debbugs.gnu.org
> Date: Wed, 03 Jun 2020 14:58:07 +0000
> 
> > I'm not sure I follow.  What do you mean by "reject"?  I thought the
> > code which ignores the metric and sets the font_not_found_p flag when
> > we get a zero-width glyph is a kind of "rejection".
> 
> I mean "treat the glyph as non-existent". Currently, for (3), glyphs
> with lbearing but no pixel width are treated as valid and expanded to
> cover a single pixel, which is all but invisible on my screen.

They are hard to spot, but if one looks close enough, IME they are
visible.

We could perhaps introduce a feature whereby such thin-space glyphs
are somehow made to stand out more, but that would be a separate
feature, because right now we have these 1-pixel thin spaces with many
control characters.

> > If you disable auto-composition-mode, and use the
> > fonts which shows CGJ as zero-width glyph, do you still see display
> > artifacts?
> 
> No.

Then I think your suggestion to handle such lgstrings as we do with
simple characters is sufficient to fix situations such as this one.

> > If not, what do you see and why is this kind of
> > 'rejection" not enough?
> 
> CGJ is displayed as a black box as wide as a space, which is perfectly
> fine. It's U+301 that's not.

Hmm... how does U+301 enter this picture?  What problems do you see
with its display?