What's best to do really depends on what the purpose is and in what exact scenario it takes place in - unfortunately you have given very little information about that.   ScrollTrigger.isScrolling() is not an eventListener per se, the way I see it - I would understand it more as a check wether ScrollTrigger is scrolling on a given event that you implemented yourself in some way.   If all you want to do is to add a class to an element whenever a user starts scrolling (and maybe