Edit in JSFiddle

function sentinelIO() {
	const sentinel = document.getElementById("sentinel");
	const header = document.getElementById("header");

	const options = {
  	root: document,
    rootMargin: "0px",
    threshold: 0,
  }
 
  const callback = (entries, observer) => {
    entries.forEach(entry => {
      if (entry.isIntersecting) {
      	console.log("entering")
        header.classList.remove('show');
        header.classList.add('hide');
      } else {
      	console.log("leaving");
       	header.classList.remove('hide');
      	header.classList.add('show');
      }
    });
  }

	observer = new IntersectionObserver(callback, options);
  
  observer.observe(sentinel);
}

sentinelIO();

<nav id="header" class="collapsible-header">
  Header
</nav>


<main id="content">
  <div id="sentinel"></div>
  <section class="my-section">
    Section 1
  </section>
    <section class="my-section">
    Section 2
  </section>
    <section class="my-section">
    Section 3
  </section>
</main>

body {
  padding: 0;
  margin: 0;
}

.hide {
  display: none;
}

.show {
  display: block;
}

.collapsible-header {
  position: sticky;
  background-color: cyan;
  width: 100%;
  top: 0;
}

#content {
  padding: 20px;
}

.my-section {
  background-color: gray;
  height: 500px;
  margin-bottom: 20px;
}

#sentinel {
  height: 1px;
  background-color: red;
}