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:

  - 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.

  - On software using HarfRust and HarfRust only, it will ligate "HarfRust" to the HarfBuzz logo.

  - On software using HarfBuzz, HarfRust, or DirectWrite, but not CoreText, it will ligate "HarfBust" to the HarfBuzz logo.

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