scroll-detection-with-window-onscroll.js

window.onscroll = function () {
    /** @type {HTMLElement | null} */
    const header = document.querySelector('header');
    if (header !== null) {
        if (window.scrollY < 5) {
            header.classList.remove('scrolled');
        } else {
            header.classList.add('scrolled');
        }
    }
};