Why reorder U+0E3A (Thai Phinthu)?

[sungsit]

Hi. Could please inform me why HarfBuzz always reorder U+0E3A (Thai Nighahit) to the lowest stack?

I can't find any particular reason why it should do that for Thai national language. Moreover, it is not desired behavior for some minority languages which use Thai script.

The screenshot above, I input Thai consonants & U+0E3A before other low vowels but HarfBuzz still displays it at the lowest position (the first 2 are actual words from Melayu-Pattani, the last one is just for testing mkmk). Compared to FontForge, it should be like a screenshot below - the font with mkmk feature can be found here https://github.com/BoonUni/boonjot).

[behdad]

Basically because Windows does that. Is that a good-enough answer?

[sungsit]

I just found out that Harfbuzz & Uniscribe override Phinthu (U+0E3A) combining class (sorry, I previously mentioned it as Nighahit) and this problem has been discussed before but I still don't know why Windows does that.

So in case someone needs to create OpenType feature to workaround this problem, there is Theppitak's guideline here (in Thai).

Thank you for creating Harfbuzz anyway, it helps doing Thai & Lao fonts a lot easier for GNU/Linux and the web!