<!DOCTYPE html>
<html reftest-async-scroll
      reftest-displayport-x="0" reftest-displayport-y="0"
      reftest-displayport-w="800" reftest-displayport-h="2000"
      reftest-async-scroll-x="0" reftest-async-scroll-y="50">
<style>
  html {
    scrollbar-width: none;
  }
  body {
    height: 4000px;
    margin: 0;
  }
  #clip {
    position: absolute;
    top: 500px;
    left: 300px;
    background: red;
    width: 400px;
    height: 400px;
    clip-path: inset(0);
  }
  #fixed {
    position: fixed;
    top: 0;
    left: 0;
    right: 0;
    bottom: 0;
    background: linear-gradient(green, blue);
  }
</style>
<div id="clip">
  <div id="fixed"></div>
</div>