Fix chaining context backtrack matching order#372
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- Backtrack arrays are stored in reverse buffer order per OT spec - Reverse before matching so backtrack[0] aligns with position before input
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The chaining-context lookup (GSUB type 6, formats 1–3) stores backtrack coverage/class/glyph arrays in reverse buffer order per the OpenType spec:
backtrack[0]is the glyph immediately before the input,backtrack[1]is one further back, etc. The current matcher feeds the stored order directly tosequenceMatches(-N, …), which iterates forward — so it ends up comparingbacktrack[0]against the position farthest from the input. When all backtrack positions accept the same coverage (the common case) the bug is invisible, but rules with distinct positions silently fail to fire.The fix reverses the backtrack array before matching, in all three chain-context formats.
Test
Added a shaping test using Noto Sans Syriac Estrangela's
caltchain rule where:backtrack[0]accepts a mark glyph (uni0731/uni0734/uni0737)backtrack[1]accepts a positional consonant (uni0712.Init/uni0712.Medi/uni071A.Medi)uni0713.Fina, substituted touni0713.FinaWideInput
ܒܱܓ(U+0712 U+0731 U+0713) joins to[uni0712.Init, uni0731, uni0713.Fina]. Per spec the rule fires and producesuni0713.FinaWide(matching HarfBuzz); without the fix fontkit leavesuni0713.Finaunsubstituted.