The onEventSelect event is fired by the JavaScript Scheduler when the event selection changes (before the default action, which is selecting/deselecting the event in the UI).
You can cancel the default action using args.preventDefault().
See also the onEventSelected event.
DayPilot.Scheduler.onEventSelect(args)args.e (DayPilot.Event) - the event reference
args.selected (boolean) - current selection state (before the change)
args.ctrl (boolean) - Ctrl key state
args.meta (boolean) - Meta key state
args.preventDefault() - cancels the default action
onEventSelect(e, change)