seedproject-web/app/src/components/TableOfContents.astro

---
const { items = [] } = Astro.props;
---

{
  items.length > 0 && (
    <div data-toc>
      <div class="text-xs font-medium uppercase tracking-wider text-muted-foreground">On this page</div>
      <ul class="mt-3 space-y-2 border-l border-border">
        {items.map((item) => (
          <li data-toc-item class="relative" style={`padding-left: ${item.level === 3 ? 24 : 12}px`}>
            <a
              href={`#${item.id}`}
              data-toc-link={item.id}
              class="block py-0.5 text-sm text-muted-foreground transition-colors hover:text-foreground"
            >
              {item.text}
            </a>
          </li>
        ))}
      </ul>
    </div>
  )
}

<style>
  [data-toc-item].is-active::before {
    background: var(--color-primary);
    bottom: 0.125rem;
    content: "";
    left: -1px;
    position: absolute;
    top: 0.125rem;
    width: 1px;
  }
</style>

<script>
  const toc = document.querySelector("[data-toc]");
  if (toc) {
    const links = [...toc.querySelectorAll("[data-toc-link]")];
    const setActive = (id) => {
      links.forEach((link) => {
        const active = link.getAttribute("data-toc-link") === id;
        link.classList.toggle("text-primary", active);
        link.classList.toggle("text-muted-foreground", !active);
        link.closest("[data-toc-item]")?.classList.toggle("is-active", active);
      });
    };
    const observer = new IntersectionObserver(
      (entries) => {
        const visible = entries.filter((entry) => entry.isIntersecting);
        if (visible[0]) setActive(visible[0].target.id);
      },
      { rootMargin: "-80px 0px -70% 0px" },
    );
    links.forEach((link) => {
      const id = link.getAttribute("data-toc-link");
      const heading = id ? document.getElementById(id) : null;
      if (heading) observer.observe(heading);
    });
  }
</script>