[HarfBuzz] Shaping Thai text

[Peter Hunter]

I'm using Harfbuzz (not Harfbuzz-ng) for shaping Thai text (using HB_BasicShape/HB_HeuristicPosition)

Not being familiar with the Thai language, I've been told that the vowels/marks are positioned "too high", (particularly compared to MS wordPad/Notepad)
At least with the fonts I have access to, the vowels/marks are already configured to an appropriate height for most situations and do not require the extra adjustment provided in "positionCluster" 

Disabling the extra adjustment however causes a problem with the positioning of the combining above marks when used in conjunction with 0xe1b,0xe1d,0xe1f characters( PO PLA, PO FA and PO FAN). (These characters all have a trailing vertical stem that overlaps the combining above marks when left at their default position)
In wordpad/notepad these marks are left at their design height, but positioned left aligned with the base character. 
(The fonts appear to have been designed for this situation, but I don't know if that is true for Thai fonts in general)

So, I'd like to know if this issue has been resolved in "-ng" or if anyone else has experienced and/or resolved this problem

Thank you for your patience
Peter Hunter