Active highlighting

On the client, TableOfContents creates an IntersectionObserver for h2[id] and h3[id] elements.

Behavior

• Root margin: 0px 0px -80% 0px (heading near the top of the viewport)
• Active items receive active-toc-item (plus top/sub variants)
• lastVisibleHeading keeps the last intersecting id as a fallback
• Observation is rebuilt when TOC data or path changes
• Cleanup runs on unmount via onUnmounted in setup

Scoping

When TOC link ids are known, only those heading ids are observed (avoids tracking unrelated page chrome headings).

Styling

.active-toc-item {
  color: #4ade80;
  font-weight: 600;
}