ScrollSpy

Navigationcore-ui/patterns/scrollspy ↗

IntersectionObserver active-section tracking for in-page anchor navs.

Note
ScrollSpy wraps a nav like the one above with scrollspy.Wrap(cfg, nav) and sets aria-current + .is-active on the link whose target is in view. It needs a tall, scrollable page region — see it working live in the left rail of any /docs/* page.