The patch simply reverses the mistake of the Unicode Technical Committee by making U+1A60 TAI THAM SAKOT have lower priority so that it comes after any tone marks in a word.
This is example illustrates one of the benefits of collaboration within the larger Open Source development community. As a “new” script in Unicode, no type layout engine had ever been thoroughly tested to see if it supported the complex layout that Tai Tham requires. While Behdad Esfahbod and the other developers of HarfBuzz had added code in HarfBuzz to support Indic and Indic-derived scripts such as Tai Tham, until a Unicode Tai Tham font became available, there was really no way to know whether HarfBuzz truly supported Tai Tham or not.
On the other side of things, for me as a developer of a Unicode Tai Tham font, I didn’t really have any OpenType-based platform that would fully support my font. So it has been a bit of a “chicken and egg” kind of problem. Fortunately, the HarfBuzz team has been very responsive and thus the problem described here was quickly resolved.
By early February, 2014, the patched version of HarfBuzz finally made it’s way into Aurora, the nightly development version of the Firefox browser. This meant that I finally had an easy-to-use “platform” that could render my OpenType-based Tai Tham font.