JSFiddle - React, Tailwind, and code Playground

HTML

<div id="divTopFixed" runat="server" style="width: 100%;position: fixed; top: 0px;background-color: lightgreen;">Some DIVs<br>That create variable height</div>
<div id="the_div" style="background-color: #EBECEF; margin-top: 40px; width:100%;">CONTENTS<br>...</div>

JavaScript

window.addEventListener('resize', (function resize(){
  document.getElementById('the_div').style.marginTop = document.getElementById('divTopFixed').offsetHeight + 'px';
  return resize;
})());