fun improvements (#1)

Reviewed-on: #1

components/table-of-contents.tsx

@@ -0,0 +1,64 @@
+"use client";
+
+import { useEffect, useState } from "react";
+import type { Heading } from "@/lib/posts";
+
+interface TableOfContentsProps {
+  headings: Heading[];
+}
+
+export default function TableOfContents({ headings }: TableOfContentsProps) {
+  const [activeSlug, setActiveSlug] = useState(() => headings[0]?.slug ?? "");
+
+  useEffect(() => {
+    if (headings.length === 0) return;
+
+    const observer = new IntersectionObserver(
+      (entries) => {
+        for (const entry of entries) {
+          if (entry.isIntersecting) {
+            setActiveSlug(entry.target.id);
+          }
+        }
+      },
+      { rootMargin: "0% 0% -70% 0%", threshold: 0 }
+    );
+
+    headings.forEach(({ slug }) => {
+      const el = document.getElementById(slug);
+      if (el) observer.observe(el);
+    });
+
+    return () => observer.disconnect();
+  }, [headings]);
+
+  if (headings.length === 0) return null;
+
+  return (
+    <nav>
+      <p className="text-xs font-bold uppercase tracking-widest text-blue-800 mb-3">
+        Contents
+      </p>
+      <ul className="space-y-1.5">
+        {headings.map(({ slug, text, level }) => (
+          <li
+            key={slug}
+            className={level === 3 ? "pl-3" : ""}
+          >
+            <a
+              href={`#${slug}`}
+              aria-current={activeSlug === slug ? "true" : undefined}
+              className={`text-sm leading-snug transition-colors duration-150 ${
+                activeSlug === slug
+                  ? "text-blue-700 font-semibold"
+                  : "text-gray-600 hover:text-blue-600"
+              }`}
+            >
+              {text}
+            </a>
+          </li>
+        ))}
+      </ul>
+    </nav>
+  );
+}