Scroll Snap State Sync

main {
    /* Enable scroll snapping on the container */  
  scroll-snap-type: y proximity;
  overflow-y: auto;
}

h2 {
  /* Define how headers align when snapped */
  scroll-snap-align: start;
}

<!-- MANDATORY: Wrap table of contents links inside a proper navigation landmark -->
<nav aria-label="Table of contents">
  <ul>
    <li><a href="#section-1" aria-current="location">Section 1</a></li>
    <li><a href="#section-2">Section 2</a></li>
  </ul>
</nav>

const main = document.getElementById('main');
const links = document.querySelectorAll('nav a');

// The event fires when the scroller settles on a new snap target
main.addEventListener('scrollsnapchange', (event) => {
  // Use snapTargetBlock for vertical or snapTargetInline for horizontal
  const snappedHeader = event.snapTargetBlock;
  
  if (snappedHeader) {
    setSelectedParagraph(snappedHeader.id);
  }
});

// Feature detect support for scroll snap events
if (!('onscrollsnapchange' in HTMLElement.prototype)) {
  const observer = new IntersectionObserver(
    () => {
      // Each time the set of intersecting headers changes, find the top
      // header that is visible.
      const topEntry = [...headers].reduce((currentTop, header) => {
        // Use the bottom to handle scrolling up, when the top is still offscreen
        const {bottom} = header.getBoundingClientRect();
        // Don't match if the header's bottom is above the scrollport
        if (bottom < 0) return;
        if (!currentTop) return header;
        return bottom <
          currentTop.getBoundingClientRect().bottom
          ? header
          : currentTop;
      }, undefined);
      if (topEntry) setSelectedParagraph(topEntry.id);
    },
    { root: main, threshold: 0.9 // Adjust based on your use case },
  );

  // Observe all snap targets (e.g., section headers)
  document.querySelectorAll('h2').forEach(header => observer.observe(header));
}