[HarfBuzz] how to detect missing glyphs e.g. for font substitition

[Richard Wordingham]

On Tue, 12 May 2015 01:13:27 +0400
Konstantin Ritt <ritt.ks at gmail.com> wrote:

> Whilst variation selectors are of Default_Ignorable_Code_Point
> property, regional indicators are not (bug in Unicode? or it was done
> intentionally?), which means invalid or unsupported VS sequence will
> be replaced with a non-advancing whitespace, however invalid or
> unsupported RI sequence will result in a run of .notdef glyphs (no
> flags, no letters).

Is there a widespread font that supports some regional indicators
but does that? It seems bizarre behaviour.  I would have expected the
individual letters to be rendered.  Obviously, if regional indicators
are not supported at all, one will get a pair of .notdef glyphs.

Richard.