<!DOCTYPE html>
<style>
  body, html {
    margin: 0;
    background-color: limegreen;
  }
  #bottombar {
    position: fixed;
    left: 0;
    bottom: 0;
    width: 100%;
    height: 50%;
    background-color: green;
    border: 10px solid black;
    box-sizing: border-box;
  }
</style>
<div id="inflow"></div>
<div id="bottombar"></div>