JSFiddle - React, Tailwind, and code Playground

HTML

<div style="width:1300px">
  <ul class="ul-length">
    <li> A </li>
    <li> B </li>
    <li> B </li>
    <li> B </li>
    <li> B </li>
    <li> B </li>

    <li class="fillgap"/>
  </ul>
</div>

CSS

.ul-length {
  position: relative;
  with:50px;
  height:50px;
  overflow: auto;
}

.fillgap {
  position:fixed;
  width:50px; 
  bottom:0;
  left: 0;
    
  /*show me*/
  height: 1px;
  background: black;
}