feat(school-detail): highlight active section in sticky nav on scroll

Uses IntersectionObserver on each section element. Multiple thresholds
(0, 0.1, 0.25, 0.5, 0.75, 1.0) track the intersection ratio of every
section simultaneously; whichever has the highest visible ratio at any
moment becomes the active item. rootMargin offsets for the sticky nav
height so a section is only considered active once it's genuinely in
view beneath the bar.

Active link gets .sectionNavLinkActive — coral background + white text,
matching the phase tab active style used elsewhere in the product.
Observer is cleaned up on unmount.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

nextjs-app/components/SchoolDetailView.tsx

@@ -73,6 +73,8 @@ export function SchoolDetailView({
   const { addSchool, removeSchool, isSelected } = useComparison();
   const isInComparison = isSelected(schoolInfo.urn);
 
+  const [activeSection, setActiveSection] = useState<string>('');
+
   const latestResults = yearlyData.length > 0 ? yearlyData[yearlyData.length - 1] : null;
 
   // Phase detection
@@ -80,6 +82,42 @@ export function SchoolDetailView({
   const isSecondary = phase.toLowerCase().includes('secondary') || phase.toLowerCase() === 'all-through';
   const isPrimary = !isSecondary;
 
+  // Track active section as user scrolls
+  useEffect(() => {
+    const ids = navItems.map(n => n.id);
+    if (!ids.length) return;
+
+    const observers: IntersectionObserver[] = [];
+
+    // We keep a map of how much each section is visible so we can pick the
+    // most-visible one when multiple overlap the viewport.
+    const ratioMap: Record<string, number> = {};
+
+    const pickActive = () => {
+      const top = Object.entries(ratioMap).sort((a, b) => b[1] - a[1])[0];
+      setActiveSection(top?.[1] > 0 ? top[0] : '');
+    };
+
+    ids.forEach(id => {
+      const el = document.getElementById(id);
+      if (!el) return;
+      ratioMap[id] = 0;
+      const obs = new IntersectionObserver(
+        ([entry]) => {
+          ratioMap[id] = entry.intersectionRatio;
+          pickActive();
+        },
+        { threshold: [0, 0.1, 0.25, 0.5, 0.75, 1.0], rootMargin: '-56px 0px 0px 0px' },
+      );
+      obs.observe(el);
+      observers.push(obs);
+    });
+
+    return () => observers.forEach(o => o.disconnect());
+  // navItems identity is stable per render — eslint-disable is intentional
+  // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, [navItems.map(n => n.id).join(',')]);
+
   // National averages (fetched dynamically so they stay current)
   const [nationalAvg, setNationalAvg] = useState<NationalAverages | null>(null);
   useEffect(() => {
@@ -314,7 +352,13 @@ export function SchoolDetailView({
           <button onClick={() => router.back()} className={styles.sectionNavBack}>← Back</button>
           {navItems.length > 0 && <div className={styles.sectionNavDivider} />}
           {navItems.map(({ id, label }) => (
-            <a key={id} href={`#${id}`} className={styles.sectionNavLink}>{label}</a>
+            <a
+              key={id}
+              href={`#${id}`}
+              className={`${styles.sectionNavLink}${activeSection === id ? ` ${styles.sectionNavLinkActive}` : ''}`}
+            >
+              {label}
+            </a>
           ))}
         </div>
       </nav>