@stephenmccall Is fillRepeaterUpcomingTrips being called from the onReady? If so, essentially the onViewportEnter event code is being declared/initiated there already. Personally, I prefer creating the event function from the property sheet with the repeater selected.
OnViewportEnter is not merely a workaround but is a good place to put that code for your intended purpose. It is doing what you want, right? With repeaters, you don’t want to execute all your code up front if you can avoid it because of load time issues.