JSFiddle - React, Tailwind, and code Playground

HTML

<header>Project Header</header> 
<main class="container" id="layout-mainContent">
    <div class="row" id="first-row">somecontent</div>
</main>

CSS

html {
    background-color: green;
    height: 100%;
 }

body {
    background-color: pink;
    height: 100%;
}

header {
   position: fixed;
   border: 1px solid red;
   top: 0px; /* NEW */
}

main {
  margin-top: 90px;
  background-color:yellow;
}