JSFiddle - React, Tailwind, and code Playground

HTML

<div class="top-strip">Header</div>
<div class="fademe">
    <h3>I want this box to be at 0% opacity once the body text hits the very top of this box</h3>
</div>
<div class="main-content">
<h2>Box should be 0% opacity by the time this reaches the top of it.</h2>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
<p>Text to simulate scrolling</p>
</div>

CSS

html,
body {
    height: 100%;
    max-width: 100%;
    margin: 0 auto;
    padding: 0;
}

div.top-strip{height:30px;display:block;border:1px solid #ccc;position:fixed;width:100%;}


div.fademe {
    width: 100%;
    position: fixed;
    top:35px;
    display:block;
    height:130px;
    background:orange;
}

div.main-content{position:relative;top:165px;}

JavaScript

var divs = $('.fademe');
$(window).on('scroll', function() {
   var st = $(this).scrollTop();
   divs.css({ 'opacity' : (1 - st/130) });
});