TimelineLite.call / TimelineLite.add(function) with zero duration about gsap HOT 1 CLOSED

This is a bit of a fuzzy area actually. Technically, an empty timeline cannot really "start" or "end" because it's empty. It must have at least 1 child to start/complete. And "start" implies the virtual playhead moving off of the very beginning spot (0) to some other time (even 0.00000001). That's what the docs indicate too, so if your timeline has a duration of zero, that means the playhead cannot move away from the beginning, so onStart shouldn't fire. See what I mean?

As you probably discovered, if you make sure there's some (even miniscule) duration, things behave the way you were anticipating. So you could place your callback at 0.00000001, for example.

from gsap.