HN added collapsible comment threads a few years old ago...modulo the meaning of "few." So there was a long time to get it right (right for HN); a lot of time f

HN added collapsible comment threads a few years old ago...modulo the meaning of "few." So there was a long time to get it right (right for HN); a lot of time for the people implementing collapsible threads to experience them as users; and in the end the implementation made minimum changes to the user experience.