Sat 11 Nov 2017 03:09:26 PM UTC, comment #6:
Using HarfBuzz 1.6.3, the candrabindu and anusvara work equally well. If the anusvara doesn’t work with some other shaper, it is probably not updated for Unicode 10. Anyway, it is not a font bug.
'abvm' doesn’t ignore mark glyphs as a special case; that is just how mark-to-base lookups work. For ⟨q dot-below dot-above⟩ you don’t need to do anything special to skip the dot below: the dot above will attach to the q as if the dot below were not there. Similarly, for ⟨consonant vowel anusvara⟩ the anusvara will attach to the consonant regardless of the vowel, assuming the vowel glyph is a mark glyph.
You should generally not change glyph classes because there is generally no reason to. When you have a reason, go for it. Characters’ glyph classes do not need to be standardized between fonts.
Revision 3754 set some vowel signs’ glyph classes explicitly. However, it skipped u_mal, uu_mal, r_voc_mal, and rr_voc_mal, which are post-base vowels too. It also set the glyph class of the pre-base and two-part vowel signs, which is unnecessary, but which I am pretty sure has no ill effects.
I don’t know why FontForge decided that the Devanagari ī should be a mark glyph but the Malayalam ī shouldn’t.
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