Yuika/assets/js/nav-collapse.js

    $(document).ready(function () {
        
        var nav = document.querySelector('.index-navbar');
        var feed = document.querySelector('.content');

        var lastScrollY = window.scrollY;
        var lastWindowHeight = window.innerHeight;
        var lastDocumentHeight = $(document).height();
        var ticking = false;

        function onScroll() {
            lastScrollY = window.scrollY;
            requestTick();
        }

        function onResize() {
            lastWindowHeight = window.innerHeight;
            lastDocumentHeight = $(document).height();
            requestTick();
        }

        function requestTick() {
            if (!ticking) {
                requestAnimationFrame(update);
            }
            ticking = true;
        }

        function update() {
            var trigger = feed.getBoundingClientRect().top + window.scrollY;
            var progressMax = lastDocumentHeight - lastWindowHeight;

            // show/hide nav
            if (lastScrollY >= trigger - 20) {
                nav.classList.add('fixed');
                nav.classList.add('nav-fixed');
                nav.classList.remove('absolute');
            } else {
                nav.classList.remove('fixed');
                nav.classList.remove('nav-fixed');
                nav.classList.add('absolute');
            }

            ticking = false;
        }

        window.addEventListener('scroll', onScroll, { passive: true });
        window.addEventListener('resize', onResize, false);

        update();

    });