JSFiddle - React, Tailwind, and code Playground

HTML

<div class="uno">
    uno
</div>
<div class="dos">
</div>

CSS

* {
    margin:0;
    padding:0;
}
body {
    background-color:#ccc;
    height:1000px;
}
.uno {
    position:relative;
    float:left;
    width:30%;
    height:200px;
    background-color:orange;
}
.dos {
    position:relative;
    float:right;
    width:70%;
    height:100px;
    background-color:purple;
}
.fixed {
    position:fixed;
    left:0;
    top:-100px;
}

JavaScript

$(window).scroll (function () {
   var st = $(this).scrollTop();
    if (st > 100) {
        $('.uno').addClass('fixed'); 
    } else {
        $('.uno').removeClass('fixed'); 
    }
})