diff --git a/wp-content/themes/apus-theme/assets/css/toc.css b/wp-content/themes/apus-theme/assets/css/toc.css
index 480732a4..596e3715 100644
--- a/wp-content/themes/apus-theme/assets/css/toc.css
+++ b/wp-content/themes/apus-theme/assets/css/toc.css
@@ -19,7 +19,9 @@
     padding: 1.5rem;
     margin: 2rem 0;
     box-shadow: 0 2px 4px rgba(0, 0, 0, 0.05);
-    position: relative;
+    position: sticky;
+    top: 5.5rem;
+    z-index: 10;
 }
 
 .apus-toc-header {
@@ -170,11 +172,13 @@
 .apus-toc-link {
     color: #212529;
     text-decoration: none;
-    display: inline-block;
-    transition: color 0.2s ease, transform 0.2s ease;
+    display: block;
+    transition: color 0.2s ease, transform 0.2s ease, border-color 0.2s ease;
     line-height: 1.5;
     position: relative;
-    padding: 0.25rem 0;
+    padding: 0.25rem 0 0.25rem 1rem;
+    border-left: 3px solid transparent;
+    margin-left: -1rem;
 }
 
 .apus-toc-link:hover {
@@ -188,22 +192,11 @@
     border-radius: 2px;
 }
 
-/* Active link highlighting */
+/* Active link highlighting - Issue #55 */
 .apus-toc-link.active {
     color: #0d6efd;
     font-weight: 600;
-}
-
-.apus-toc-link.active::after {
-    content: '';
-    position: absolute;
-    left: -1rem;
-    top: 50%;
-    transform: translateY(-50%);
-    width: 4px;
-    height: 100%;
-    background-color: #0d6efd;
-    border-radius: 2px;
+    border-left-color: #0d6efd;
 }
 
 /* ========================================
@@ -348,16 +341,16 @@ h3[id] {
 }
 
 .apus-toc-list::-webkit-scrollbar-thumb {
-    background: #888;
+    background: #cbd5e0;
     border-radius: 3px;
 }
 
 .apus-toc-list::-webkit-scrollbar-thumb:hover {
-    background: #555;
+    background: #a0aec0;
 }
 
 /* Firefox */
 .apus-toc-list {
     scrollbar-width: thin;
-    scrollbar-color: #888 #f1f1f1;
+    scrollbar-color: #cbd5e0 #f1f1f1;
 }
diff --git a/wp-content/themes/apus-theme/assets/js/toc.js b/wp-content/themes/apus-theme/assets/js/toc.js
index 20586107..4426e9c8 100644
--- a/wp-content/themes/apus-theme/assets/js/toc.js
+++ b/wp-content/themes/apus-theme/assets/js/toc.js
@@ -94,7 +94,7 @@
                 }
 
                 // Update active state
-                updateActiveLink(this);
+                updateActiveLinkOnClick(this);
 
                 // Focus the target heading for accessibility
                 targetElement.setAttribute('tabindex', '-1');
@@ -104,7 +104,8 @@
     }
 
     /**
-     * Initialize active link highlighting based on scroll position
+     * Initialize active link highlighting with IntersectionObserver
+     * Issue #55 - ScrollSpy implementation
      */
     function initActiveHighlight() {
         const tocLinks = document.querySelectorAll('.apus-toc-link');
@@ -114,51 +115,67 @@
             return;
         }
 
-        let ticking = false;
+        // Keep track of which headings are currently visible
+        const visibleHeadings = new Set();
 
-        // Debounced scroll handler
-        function onScroll() {
-            if (!ticking) {
-                window.requestAnimationFrame(function() {
-                    updateActiveOnScroll(headings, tocLinks);
-                    ticking = false;
-                });
-                ticking = true;
-            }
-        }
+        // IntersectionObserver configuration with custom rootMargin
+        // -20% top, -35% bottom for optimal detection
+        const observerOptions = {
+            root: null, // viewport
+            rootMargin: '-20% 0px -35% 0px',
+            threshold: 0
+        };
 
-        window.addEventListener('scroll', onScroll, { passive: true });
+        // Callback when heading visibility changes
+        const observerCallback = function(entries) {
+            entries.forEach(function(entry) {
+                const headingId = entry.target.id;
 
-        // Initial update
-        updateActiveOnScroll(headings, tocLinks);
+                if (entry.isIntersecting) {
+                    visibleHeadings.add(headingId);
+                } else {
+                    visibleHeadings.delete(headingId);
+                }
+            });
+
+            // Update active link based on visible headings
+            updateActiveLink(visibleHeadings, tocLinks, headings);
+        };
+
+        // Create observer
+        const observer = new IntersectionObserver(observerCallback, observerOptions);
+
+        // Observe all headings
+        headings.forEach(function(heading) {
+            observer.observe(heading);
+        });
     }
 
     /**
-     * Update active link based on scroll position
+     * Update active link based on visible headings
      *
-     * @param {Array} headings Array of heading elements
+     * @param {Set} visibleHeadings Set of currently visible heading IDs
      * @param {NodeList} tocLinks TOC link elements
+     * @param {Array} headings Array of all heading elements
      */
-    function updateActiveOnScroll(headings, tocLinks) {
-        const scrollPosition = window.scrollY + 100; // Offset for better UX
+    function updateActiveLink(visibleHeadings, tocLinks, headings) {
+        // If no headings are visible, don't change anything
+        if (visibleHeadings.size === 0) {
+            return;
+        }
 
-        // Find the current heading
-        let currentHeading = null;
-        for (let i = headings.length - 1; i >= 0; i--) {
-            if (headings[i].offsetTop <= scrollPosition) {
-                currentHeading = headings[i];
+        // Find the first visible heading (topmost in document order)
+        let activeHeading = null;
+        for (let i = 0; i < headings.length; i++) {
+            if (visibleHeadings.has(headings[i].id)) {
+                activeHeading = headings[i];
                 break;
             }
         }
 
-        // If we're at the top, use the first heading
-        if (!currentHeading && scrollPosition < headings[0].offsetTop) {
-            currentHeading = headings[0];
-        }
-
-        // Update active class
+        // Update active class on TOC links
         tocLinks.forEach(function(link) {
-            if (currentHeading && link.getAttribute('href') === '#' + currentHeading.id) {
+            if (activeHeading && link.getAttribute('href') === '#' + activeHeading.id) {
                 link.classList.add('active');
             } else {
                 link.classList.remove('active');
@@ -171,7 +188,7 @@
      *
      * @param {Element} clickedLink The clicked TOC link
      */
-    function updateActiveLink(clickedLink) {
+    function updateActiveLinkOnClick(clickedLink) {
         const tocLinks = document.querySelectorAll('.apus-toc-link');
         tocLinks.forEach(function(link) {
             link.classList.remove('active');
@@ -198,7 +215,7 @@
                     behavior: 'smooth',
                     block: 'start'
                 });
-                updateActiveLink(tocLink);
+                updateActiveLinkOnClick(tocLink);
             }, 100);
         }
     }