JSFiddle - React, Tailwind, and code Playground

HTML

<div id="main">
  <div id="topbar">
    <div id="topbar-spacer">
      &nbsp;
    </div>
    <div id="topbar-content">
      This is our topbar
    </div>
  </div>
  <div id="sidenav">
    This is our side nav
  </div>
  <div id="maincontent">
    This is our main content
  </div>

</div>

CSS

/* Just for testing */
#main {  
  height: 100vh;
}

#topbar {
  background: #ad1457;
  color: #eeeeee;
  height: 50px;
}

#topbar-spacer {
  width: 200px;
  float: left;
}

#topbar-content {
  border-left: 3px solid #26a69a;
  height: 100%;
  background: #0277bd;
  border-radius: 40px 0 0;
  padding-left: 30px;
  box-sizing: border-box;
  text-align: center;
  overflow: hidden;
}

#sidenav {
  width: 200px;
  height: 100%;
  background: #ad1457;
  color: #eeeeee;
  float: left;
  border-right: 3px solid #26a69a;
  box-sizing: content-box;
}