JSFiddle - React, Tailwind, and code Playground

HTML

<div id="expand-box-header" style="display:flex">
      <div style="display:inline; height: 100%; border-right: 1px solid black; top: 0px; bottom:0px; background-color: red; align: center">Top Left header</div>
      <div style="display:initial;">
        <div style="background-color: blue">Line 1</div>
        <div style="background-color: green">Line 2</div>
      </div>
     
   </div>

CSS

#expand-box
{
    /* width: 100%; */
    padding: 0;
    border: 2px solid #BBB;
    margin: 7px 0 0 0;
}

#expand-box-header
{
    background-color: #BBB;    margin: 0;
    color: #FFF;
    padding: 0 0 3px 2px;
}