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

[Pip Cet <pipcet@gmail.com>]

On Mon, Jun 1, 2020 at 7:48 PM Pip Cet <pipcet@gmail.com> wrote:
> > > Indeed, the composition gstring is a single zero-width glyph.
> > See the composition information above: my interpretation of it is that
> > the composed glyph is not zero-width.
>
> ... something is odd here, I agree.

I think it's a very odd combination of things:
1. a font which defines an isolated CGJ to have zero width
2. an isolated CGJ appearing in the first place (in this case, because
another font does not support CGJ)
3. the fall-back [nil 0 compose-gstring-for-graphic] rule defined for
codepoint #x34f
4. compose-gstring-for-graphic attempting to salvage non-spacing
characters not following base characters, and producing zero-width
lgstrings from zero-width lglyphs

Avoiding any of the four will avoid the problem. (1) is something we
cannot fix directly. (2) is also something that a user may want. (3)
could be dropped, and (4) could be expanded to take care of the
zero-width case.

However, as long as zero-width gstrings can somehow slip through, I
suggest we also apply the patch I sent, assuming it fixes the problem.

We might consider simply prohibiting zero-width zero-lbearing
zero-rbearing gstrings, the way we prohibit zero-width zero-lbearing
zero-rbearing characters in the code I posted.