[HarfBuzz] OpenType "mark" Ligatures ... ?

[Ed Trager]

Hi, Behdad and OpenType Experts-at-Large!

I'm trying to find a good OpenType solution to a problem in the Tai Tham
font that I am developing.

The problem, as illustrated in (1) in the attached image, is that the
subjoined form of consonant u1A36 TAI THAM LETTER NA or subjoined form of
consonant u1A3E TAI THAM LETTER MA may commonly be followed by a subjoined
dependent vowel u1A69 TAI THAM VOWEL SIGN U or u1A6A TAI THAM VOWEL SIGN UU
and we don't want these to overlap.

The solution that I thought would work quite well is to create ligature
glyphs with the correct spacing, as shown in (2) in the attached figure.
Note that these "ligature" glyphs are "mark" glyphs

The lookup table, as one would expect it to be, is shown in (3).

However, this apparently does not work!

Perhaps the reason has to do with what Matthew Scala, in his blog at
http://ansuz.sooke.bc.ca/entry/131 says ? :

"It appears that if you use a glyph as a "mark" in the GPOS table, then you
are not allowed to use it at all in the GSUB table. This is because every
glyph must be categorized as at most one of "base," "ligature," "mark," or
"component" in the (usually implicit) GDEF table, a fact which is alluded
to rather vaguely in subsection 9.b of the specification. FontForge allows
putting "mark" glyphs in substitutions, but not really. The GSUB table ends
up full of garbage if you try, and it segfaults when you save. Apparently,
some of the Adobe tools I don't have will fail in a more informative way. I
don't see a good reason for marks in substitutions not to be allowed, and
it would be useful if they were allowed, but it seems clear that they are
not allowed."

OK, Dear OpenType Experts, If my solution is not workable, then please
enlighten me: ¿¿¿What is the BEST OpenType solution to this sort of problem
???
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