[HarfBuzz] Why mark glyphs are skipped when MarkBasePos matching?

[Shusaku KIMURA]

Dear developer team.

I have a question about MarkBasePos matching behavior.  It seems that
all glyphs classified as mark by GDEF Glyph Class
Definition Table are skipped when MarkBasePos matching.

For example, the following glyph sequence is input.  B is a glyph in
Base Coverage Table, M is a glyph in Mark Coverage Table
and m is a mark glyph defined by GDEF.

    B m M

The implementation of HarfBuzz finds B as base of M even though there
is m between B and M. If m is not a mark glyph, B and M
are not matched. I tried to find out the reason of the behavior in the
specification, but could not.  Can anyone teach me why
such behavior is needed?  The link to the related discussion log is enough.

Thank you,

-- 
Shusaku KIMURA
skimura0 at gmail.com