iron-scroll-spy is a container for a vertical list of elements, it observers the scroll in the scroll-target (if not specified, default to iron-scroll-spy itself) and select an element when it pass the middle of the scroll-target viewport.
Using iron-auto-scroll-spy, it also listen to change of the selected attribute and automatically scroll to the selected item.
Check out the 2.0-preview branch for Polymer 2!
See component page
bower i -S Zecat/iron-scroll-spy