HarfBust: a font software puzzle for the curious

I have a puzzle for the hacker in you: The font HarfBust.ttf is an offshoot of Anton Regular, with a twist:

1. On software using HarfBuzz (as far back as you go, back to the 2000s) and HarfBuzz only, it will ligate "HarfBuzz" to the HarfBuzz logo.
2. On software using HarfRust and HarfRust only, it will ligate "HarfRust" to the HarfBuzz logo.
3. On software using HarfBuzz, HarfRust, or DirectWrite, but not CoreText, it will ligate "HarfBust" to the HarfBuzz logo.

Here is sample text to showcase the behavior of your browser:

1. Is it HarfBuzz? HarfBuzz
2. Is it HarfRust? HarfRust
3. Is it HarfBust? HarfBust

How does this work? The font does not use any non-standard OpenType features like `HARF`/`Harf`/`BUZZ`/`Buzz` (which HarfBuzz / HarfRust apply).

Enjoy,
--behdad