I used a JS solution. It works in Firefox and Chrome. Any problems, let me know.
html
<body>
<header id="header">
<h1>Extra-Long Page Heading That Wraps</h1>
<nav id="nav">
<ul>
<li><a href="" title="">Home</a></li>
<li><a href="" title="">Page 2</a></li>
<li><a href="" title="">Page 3</a></li>
</ul>
</nav>
</header>
<main>
<p><!-- ridiculously long content --></p>
</main>
<footer>
<p>FOOTER CONTENT</p>
</footer>
<script src="navbar.js" type="text/javascript"></script>
</body>
css
nav a {
background: #aaa;
font-size: 1.2rem;
text-decoration: none;
padding: 10px;
}
nav a:hover {
background: #bbb;
}
nav li {
background: #aaa;
padding: 10px 0;
}
nav ul {
background: #aaa;
list-style-type: none;
margin: 0;
padding: 0;
}
@media (min-width: 768px) {
nav ul {
display: flex;
}
}
js
function applyNavbarSticky() {
let header = document.querySelector('body > header:first-child')
let navbar = document.querySelector('nav')
header.style.position = 'sticky'
function setTop() {
let headerHeight = header.clientHeight
let navbarHeight = navbar.clientHeight
let styleTop = navbarHeight - headerHeight
header.style.top = `${styleTop}px`
}
setTop()
window.onresize = function () {
setTop()
}
}