<!DOCTYPE html>
<style>
  body,html {
    margin: 0;
    width: 100%;
    height: 100%;
  }

  .sticky {
    background: green;
    position: sticky;
    bottom: 50vh;
    width: 100px;
    height: 10%;
  }

  .spacer {
    height: 90%;
  }
  .long {
    height: 600vh;
  }

  .position-parent {
    position: absolute;
    display: flex;
    align-items: center;
    justify-content: center;
    width: 100%;
    height: 100%;
    top: 100vh;
    background-color: lightgrey;
  }

  .container {
    width: 100px;
    height: 100%;
    background-color: grey;
  }

  button {
    position: fixed;
    left: 20px;
    top: 20px;
  }

  .fixed {
    position: fixed;
    top: 25vh;
  }
</style>

<div class="position-parent">
  <div class="container">
    <div class="spacer"></div>
    <div class="sticky"></div>
  </div>
</div>
<div class="long"></div>
<button id="button">Toggle Fixed</button>
</html>