feat: highlight active heading in sidebar table of contents

src/App.tsx

@@ -343,6 +343,7 @@ function App() {
   const [selectionRects, setSelectionRects] = useState<{ left: number; top: number; width: number; height: number }[]>([]);
   const [tabFocusIndex, setTabFocusIndex] = useState(0);
   const [sidebarFocusIndex, setSidebarFocusIndex] = useState(0);
+  const [activeHeadingText, setActiveHeadingText] = useState<string | null>(null);
 
   const contentRef = useRef<HTMLDivElement>(null);
   const sidebarRef = useRef<HTMLDivElement>(null);
@@ -1033,6 +1034,35 @@ function App() {
     }
   }, [tabs.length, tabFocusIndex]);
 
+  // Active heading tracking via IntersectionObserver
+  useEffect(() => {
+    if (!activeTab || headings.length === 0) {
+      setActiveHeadingText(null);
+      return;
+    }
+
+    const allHeadings = document.querySelectorAll(
+      '.markdown-content h1, .markdown-content h2, .markdown-content h3, .markdown-content h4, .markdown-content h5, .markdown-content h6'
+    );
+
+    if (allHeadings.length === 0) return;
+
+    const observer = new IntersectionObserver(
+      (entries) => {
+        for (const entry of entries) {
+          if (entry.isIntersecting) {
+            setActiveHeadingText(entry.target.textContent?.trim() || null);
+            break;
+          }
+        }
+      },
+      { rootMargin: '0px 0px -70% 0px', threshold: 0.1 }
+    );
+
+    allHeadings.forEach(h => observer.observe(h));
+    return () => observer.disconnect();
+  }, [activeTab, headings, highlightedHtml]);
+
   // Reset sidebar focus index when headings change
   useEffect(() => {
     setSidebarFocusIndex(0);
@@ -1156,7 +1186,7 @@ function App() {
                       <button
                         key={h.id}
                         ref={(el) => { sidebarItemRefs.current[index] = el; }}
-                        className={`sidebar-item h${h.level}`}
+                        className={`sidebar-item h${h.level}${h.text === activeHeadingText ? ' active' : ''}`}
                         tabIndex={index === sidebarFocusIndex ? 0 : -1}
                         onClick={() => { setSidebarFocusIndex(index); scrollToHeading(h.text); }}
                       >