This example shows a navigation menu that automatically highlights the section currently visible in the viewport as the user scrolls. The code uses jQuery to listen for window scroll and resize events, then checks each heading’s offset position against the viewport bounds to determine which one is active. It applies “active” and “first” CSS classes to corresponding navigation links via `$('#nav a')`, and it runs once on page load. The data comes from the headings’ `id` attributes in the document, as seen in the GeoJSON specification page.