JSFiddle - React, Tailwind, and code Playground
HTML
<ul id="menu">
<li>Toto</li>
<li>Toto</li>
<li>Toto</li>
<li>Toto</li>
<li>Toto</li>
<li>Toto
<ul id="submenu">
<li>Tutu</li>
<li>Tutu</li>
<li>Tutu</li>
<li>Tutu</li>
<li>Tutu</li>
<li>Tutu</li>
</li>
</ul>CSS
#menu li, #submenu li {float: left; display: block;}
#menu {background: blue;overflow:hidden;height: 50px;position: relative;z-index:999;}
#submenu {background:red;position: absolute;top:0px;left:-100px;z-index: -999;}