bug#33729: 27.0.50; Partial glyphs not rendered for Gujarati with Harfbuzz enabled (renders fine using m17n)

[Khaled Hosny]

On Thu, Dec 13, 2018 at 03:25:16PM -0500, Kaushal Modi wrote:
> >
> > Screenshot to explain this issue: https://i.imgtc.com/md9Yz7X.png
> >
> 
> I don't know Arabic. But from that same screenshot, it's evident that the
> rendering of that same text is quite different between m17n and harfbuzz.

The HarfBuzz rendering of Arabic is the correct one in this screenshot.
For debugging the such rendering differences, the actual font used by
Emacs for a given part of the text need to be known, then the text and
the font can be checked against vanilla HarfBuzz (e.g. using the hb-view
command line tool); if it gives the same rendering then it is either a
HarfBuzz or font issue, if not then it is a bug in the HarfBuzz
integration code in Emacs.