[HarfBuzz] uniscribe confusion

[Martin Hosken]

Dear Behdad,

This is default shaping. The u1037 is attached to the u102F and we get the following results:

* uniscribe: [u1000=0+1002|u103D=1 at -55,0+0|u102F=2+147|u1037=3 at 217,0+0]
* harfbuzz: [u1000=0+1002|u103D=0 at -55,0+0|u102F=0+147|u1037=0 at 217,0+0]

which makes sense. Now if we introduce an increase of the advance on the u102F via a feature rule: u102F' 272 u1037 we get:

* uniscribe: [u1000=0+1002|u103D=1 at -55,0+0|u102F=2+419|u1037=3 at 217,0+0]
* harfbuzz: [u1000=0+1002|u103D=0 at -55,0+0|u102F=0+419|u1037=0 at -55,0+0]

I'm wondering if harfbuzz is over compensating differently to how uniscribe does it. It looks like uniscribe just slaps on the extra advance and doesn't compensate its attached components. I don't really mind what results I get so long as they are consistent.

Any thoughts?

Yours,
Martin