JSFiddle - React, Tailwind, and code Playground

HTML

<div id="header-wrap">
</div>
<div id="container">    
    <p>test</p>  
    <p>test</p>
    <p>test</p>
    <p>test</p>
    <p>test</p>
    <p>test</p>
    <p>test</p>
    <p>test</p>
</div>

CSS

#header-wrap {
    position: fixed;
    height: 100px;
    top: 0;
    width: 100%;
    z-index: 100;
    background-color:blue;
}
#container{ 
    margin-top: 100px;
}

JavaScript

$(window).scroll(function() {
    if($(this).scrollTop() == 0)
        $('#header-wrap').css('opacity','1');
    else
        $('#header-wrap').css('opacity','.4');
});