function toggleTheme() {
            document.body.classList.toggle('dark-mode');
            const isDark = document.body.classList.contains('dark-mode');
            document.getElementById('theme-icon-sun').style.display = isDark ? 'block' : 'none';
            document.getElementById('theme-icon-moon').style.display = isDark ? 'none' : 'block';
        }
        function toggleMenu() {
            const sidebar = document.getElementById('sidebar');
            sidebar.classList.toggle('active');
        }

        // Progress bar
        const mainEl = document.getElementById('main');
        const progressBar = document.getElementById('progress-bar');
        mainEl.addEventListener('scroll', function() {
            const scrollTop = mainEl.scrollTop;
            const scrollHeight = mainEl.scrollHeight - mainEl.clientHeight;
            const progress = scrollHeight > 0 ? (scrollTop / scrollHeight) * 100 : 0;
            progressBar.style.width = progress + '%';
        });

        // Slide tracking state
        const slides = document.querySelectorAll('.slide');
        const slidesArray = Array.from(slides);
        const sidebarLinks = document.querySelectorAll('#sidebar a');
        const slideIndicator = document.getElementById('slide-indicator');
        const totalSlides = slides.length;
        let currentSlideIndex = 0;

        function updateIndicator(index) {
            currentSlideIndex = index;
            slideIndicator.textContent = (index + 1) + ' / ' + totalSlides;
        }
        updateIndicator(0);

        // Single IntersectionObserver for sidebar active link + slide indicator
        const slideObserver = new IntersectionObserver(function(entries) {
            entries.forEach(function(entry) {
                if (entry.isIntersecting) {
                    const id = entry.target.id;
                    sidebarLinks.forEach(function(link) {
                        link.classList.remove('active');
                        if (link.getAttribute('href') === '#' + id) {
                            link.classList.add('active');
                        }
                    });
                    const idx = slidesArray.indexOf(entry.target);
                    if (idx >= 0) updateIndicator(idx);
                }
            });
        }, { root: mainEl, threshold: 0.3 });
        slides.forEach(function(slide) { slideObserver.observe(slide); });

        // Fade-in animation for slide content
        const fadeObserver = new IntersectionObserver(function(entries) {
            entries.forEach(function(entry) {
                if (entry.isIntersecting) entry.target.classList.add('visible');
            });
        }, { root: mainEl, threshold: 0.1 });
        document.querySelectorAll('.slide .content').forEach(function(el) { fadeObserver.observe(el); });

        // Sidebar collapse (always open by default)
        function toggleSidebar() {
            const sidebar = document.getElementById('sidebar');
            const btn = document.getElementById('sidebar-toggle');
            sidebar.classList.toggle('collapsed');
            btn.innerHTML = sidebar.classList.contains('collapsed') ? '»' : '«';
        }

        // Keyboard navigation
        document.addEventListener('keydown', function(e) {
            if (e.key === 'ArrowDown' || e.key === 'ArrowRight' || e.key === 'PageDown') {
                e.preventDefault();
                slidesArray[Math.min(currentSlideIndex + 1, slidesArray.length - 1)].scrollIntoView({ behavior: 'smooth' });
            } else if (e.key === 'ArrowUp' || e.key === 'ArrowLeft' || e.key === 'PageUp') {
                e.preventDefault();
                slidesArray[Math.max(currentSlideIndex - 1, 0)].scrollIntoView({ behavior: 'smooth' });
            } else if (e.key === 'Home') {
                e.preventDefault();
                currentSlideIndex = 0;
                slidesArray[0].scrollIntoView({ behavior: 'instant' });
            } else if (e.key === 'End') {
                e.preventDefault();
                currentSlideIndex = slidesArray.length - 1;
                slidesArray[slidesArray.length - 1].scrollIntoView({ behavior: 'instant' });
            } else if (e.key === 's' || e.key === 'S') {
                e.preventDefault();
                toggleSidebar();
            } else if (e.key === 'd' || e.key === 'D') {
                e.preventDefault();
                toggleTheme();
            }
        });