<!DOCTYPE html>
<title>Add background-attachment:fixed during smooth scroll</title>
<style>
#container {
  width: 200px;
  height: 200px;
  overflow: scroll;
  background: linear-gradient(green, blue);
  background-attachment: fixed;
}
#content {
  width: 7500px;
  height: 7500px;
}
</style>
<div id="container">
  <div id="content">Content</div>
</div>
<script>
container.scrollTop = 6000;
</script>