Add scroll-synced active state to post TOC

components/post-toc.tsx

@@ -10,6 +10,7 @@ interface TocItem {
 
 export function PostToc() {
   const [items, setItems] = useState<TocItem[]>([]);
+  const [activeId, setActiveId] = useState<string | null>(null);
 
   useEffect(() => {
     const headings = Array.from(
@@ -23,6 +24,28 @@ export function PostToc() {
         depth: el.tagName === 'H3' ? 3 : 2
       }));
     setItems(mapped);
+
+    const observer = new IntersectionObserver(
+      (entries) => {
+        entries.forEach((entry) => {
+          if (entry.isIntersecting) {
+            const id = (entry.target as HTMLElement).id;
+            if (id) {
+              setActiveId(id);
+            }
+          }
+        });
+      },
+      {
+        // Trigger when heading is in upper 40% of viewport
+        rootMargin: '0px 0px -60% 0px',
+        threshold: 0.1
+      }
+    );
+
+    headings.forEach((el) => observer.observe(el));
+
+    return () => observer.disconnect();
   }, []);
 
   if (items.length === 0) return null;
@@ -37,7 +60,11 @@ export function PostToc() {
           <li key={item.id} className={item.depth === 3 ? 'pl-3' : ''}>
             <a
               href={`#${item.id}`}
-              className="line-clamp-2 hover:text-blue-600 dark:hover:text-blue-400"
+              className={`line-clamp-2 hover:text-blue-600 dark:hover:text-blue-400 ${
+                item.id === activeId
+                  ? 'text-blue-600 dark:text-blue-400 font-semibold'
+                  : ''
+              }`}
             >
               {item.text}
             </a>