[HarfBuzz] Tai Tham / Lanna (iso15924="lana") shaping question

[Ed Trager]

Hi, Behdad and HarfBuzz folks,

Now that I have completed most of the glyphs in my "Hariphunchai" Tai
Tham font, I'm beginning to look at HarfBuzz-ng OpenType shaping to
support my font and to support Tai Tham fonts in general.

The first area of investigation is prefixed dependent vowel symbols.
In Tai Tham, these are the 5 letters U+1A6E → U+1A72
( specifically: U+1A6E TAI THAM VOWEL SIGN E, U+1A6F TAI THAM VOWEL
SIGN AE, U+1A70 TAI THAM VOWEL SIGN OO, U+1A71 TAI THAM VOWEL SIGN AI,
and U+1A72 TAI THAM VOWEL SIGN THAM AI ).

My initial look at this today strongly suggests that the latest
HarfBuzz-ng  will only "shift" these dependent vowel signs over to the
left by a *SINGLE* base consonant:  HarfBuzz appears to have no
knowledge about *consonant clusters* in Tai Tham script.

Here is an example:  For the purpose of this email, let's suppose:

* "E" represents U+1A6E VOWEL SIGN E
* "P" represents U+1A38 CONSONANT HIGH PA
* "L" represents U+1A56  CONSONANT SIGN MEDIAL LA

In the logical backstore of the string, we type "PLE" (sounds
something similat to the English word "play").

In Tai Tham, U+1A6E VOWEL SIGN E needs to be shifted all the way to
the left so that the final visual appearance would be:

"EPL"

(in this email we will ignore the fact that in reality the "L" needs
to be subjoined and hang below the "P")

But what I get from HarfBuzz is only this:

"PEL"

... which is of course wrong.

Can someone please confirm that, based on the current code, the
expected behavior of HarfBuzz at this point in time cannot do anything
other than what I have just described?