Thanks for looking in to this. In the current codepen my last two lines of javascript are intended to clear the timeline when the the window matches the query but it is currently preventing the timeline from working at all.
When you remove the if statement, at narrow viewport i am animating an mask-imge to reveal the nav on mobile, unintentionally thought, the animation is affecting the nav's opacity at wider viewports (min-width: 40em).
Im trying to clear the timeline for wider viewports. Hope that makes sense?