Anne-Marie Concepción of InDesign Secrets sent me this question:
Interesting that kerning between the letters of a ligature breaks it, but
selecting all the letters in the ligature and tracking them in does not, at least not until it’s a significant amount. Why?
According to our composition engineer:
Neither justification or tracking will break ligatures until a hard-coded distance threshold has been exceeded. Kerning, however, is considered to be an adjustment applied to a particular combination of characters–in some cases represented by a ligatur–directly and specifically, and therefore it gets treated differently.