[HarfBuzz] Possible bug in chain_context_lookup with Gpos

[Keith Stribley]

Hi Behdad,

I think there maybe a bug in the handling of kerns using format 3 Chaining Context lookups. When GPOS lookups are used, then the output part of the buffer does not seem to be used. In this case if there is a backtrack context, then the check (c->buffer->out_len<  backtrackCount) at the start of chain_context_lookup will always fail. For GPOS I think it should use the main buffer not the output buffer. Presumably the current code works fine for Chaining Contexts for GSUB lookups, but the attached patch may be necessary for GPOS.

This patch fixes the positioning of ့ in sequences like ကဲ့ for the Padauk font for example, so that HBNG then behaves the same as other renderers like ICU and Uniscribe.

thanks,
Keith


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