https://bugs.documentfoundation.org/show_bug.cgi?id=160634

--- Comment #14 from Y. Kawara <[email protected]> ---
Manual of HarfBuzz says "If a font has an AAT morx table, then it is used for
substitutions; if not, but there is a GSUB table, then the GSUB table is used."

According to FontForge's OT->AAT equivalence for substitution:

 Single -> Non-Contextual Glyph
 Multiple -> N/A
 N/A -> Glyph Insertion
 Alternate -> N/A
 Ligature -> Ligature
 N/A -> Contextual Glyph
 Chaining / Chaining Context -> N/A
 Reverse Chaining Context -> N/A
 N/A -> Indic Rearrangement

That boils down to 9 unique types of substitutions, some only available in OT,
some only in AAT, some in both OT and AAT.

A preference for AAT becomes problematic if the right side of the list above
has "N/A". In my test case, I use Alternate type, which is only available in
OT.

Removing AAT tables completely as suggested would make it impossible to use
those substitution types that have N/A on the left side.

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