JSFiddle - React, Tailwind, and code Playground

HTML

<header class="mainHeader">
  <h1>Page Title</h1>
    <nav class="secondaryNav">
      <a href="home.htm"><img class="backButton" src="img/back2.png" alt="Back Button"></a>
      <a href="home.htm"><img class="homeButton" src="img/home.png" alt="Home Button"></a>
    </nav>
    <aside><p id="countdown"></p></aside>
</header>

CSS

*{
    height: 1000px;
}
.mainHeader h1 {
    position: fixed;
    top: 0;
    width: 100%;
    background: #ea594e;
    box-shadow: 0 0 3px 3px #888888;
    font-family: verdana;
    font-variant: small-caps;
    color: white;
    border-bottom: 1px solid black;
    margin: 0;
    padding-bottom: 0.5%;
    text-align: center;
}

.mainHeader .secondaryNav {
    background: #ffcda4;
    box-shadow: 0 3px 3px 1px #888888;
    border: 1px solid;
    border-top: none;
    width: 400px;
    position: fixed;
    top: 49px;
    left: 50%;
    margin-left: -200px;
    border-radius: 0 0 50px 50px;
    padding-top: 5px;
}

JavaScript

$('.secondaryNav').hide();
$(function() {
    $(document).on("mousewheel", function() {
        if($(document).scrollTop() > 100){
            $('.secondaryNav').show();
            $('.mainHeader h1').hide();
        } else {
            $('.secondaryNav').hide();
            $('.mainHeader h1').show();
        }
        
    });
});