[HarfBuzz] Better handling variation selectors

[Khaled Hosny]

On Sat, Apr 20, 2013 at 10:49:26AM +0100, Jonathan Kew wrote:
> On 19/4/13 22:13, Behdad Esfahbod wrote:
>
> >   - What should HB_BUFFER_FLAG_PRESERVE_DEFAULT_IGNORABLES do to variation
> >selectors?  Right now they do not become visible no matter what.  Is it
> >relevant whether the variation selector was actually used during glyph lookup
> >or not?
> 
> Yes, I think it's relevant. A variation selector that was used
> during glyph lookup has (in a sense) become "visible", manifested as
> a particular choice of glyph. I don't think it should -also- appear
> as a separate glyph, even in a "show invisibles" mode. In effect,
> the <base, VS> pair has been ligated - it's just a "ligation" that's
> handled by the cmap instead of GSUB.

But this can be said about many other control characters, ZWJ or ZWNJ
for example, they have a visual effect.

IMO PRESERVE_DEFAULT_IGNORABLES should simply show any default ignorable
character that is being removed from the output not as a result of
OpenType glyph substitution. One use of such feature (the only use?) is
to make visible all characters that has mysterious effects on the output
so they can be easily checked/edited, and I can see variation selectors
benefiting from this just like any other default ignorable.

Regards,
Khaled