<!DOCTYPE html>
<style>
.compositedBehind {
  width: 500px;
  height: 500px;
  background-color: cyan;
}

.containerOverlapsComposited {
  position: absolute;
  top: 20px;
  left: 20px;
  width: 100px;
  height: 30000px;
  background-color: green;
}

.fixed {
  position: relative;
  top: 125px;
  left: 25px;
  background-color: lime;
  width: 50px;
  height: 50px;
}

</style>

<script>
onload = () => {
  window.scrollTo(0, 100);
};
</script>

<div class="compositedBehind"> </div>

<div class="containerOverlapsComposited">
  <div class="fixed"></div>
</div>