[HarfBuzz] Regression in ZWJ handling for Indic CV ligatures

[Ian-Mathew Hornburg]

2013/4/4 Behdad Esfahbod <behdad at behdad.org>:
> To add more context here, there's an inherent conflict between general Unicode
> principle that inserting a ZWJ should encourage ligation, and the use of ZWJ
> in most of Indic parts of the standard to inhibit conjunct formation. We
> tried to implement this in HarfBuzz, by transparently skipping ZWJ in
> "presentation forms" features, but respecting its presence to inhibit ligation
> in "basic shaping features". However, fonts in common circulation use (what I
> call) arbitrary features to perform ligations and substitutions, which as a
> result can behave incorrectly when, say, a font uses 'psts' instead of 'pstf'
> to form a subjoined conjunct. *That* is why we had to revert this change.

So what you're saying is that because Indic ZWJ behavior is different
than the general Unicode principle, it can't be automagically handled
by HarfBuzz, and instead that the *fonts themselves* that need to
contain lookups to handle ZWJs for special shaping cases?

Am I understanding that correctly?