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chromium/third_party/blink/web_tests/external/wpt/intersection-observer/fixed-position-child-scroll.html 

<!doctype html>
<meta name="viewport" content="width=device-width,initial-scale=1">
<script src="/resources/testharness.js"></script>
<script src="/resources/testharnessreport.js"></script>
<script src="./resources/intersection-observer-test-utils.js"></script>
<div style="position: fixed; top: 0; left: 0">
  <div id="target" style="width: 100px; height: 100px"></div>
</div>
<div id="log" style="height: 2000px"></div>
<script>
var entries = [];

runTestCycle(function() {
  var observer = new IntersectionObserver(function(changes) {
    entries = entries.concat(changes)
  });
  observer.observe(target);
  entries = entries.concat(observer.takeRecords());
  assert_equals(entries.length, 0, 'No initial notifications.');
  runTestCycle(step0, 'First rAF.');
}, 'Fixed-position intersection observer on scroll');

function step0() {
  window.scrollTo(0, 1000);
  runTestCycle(step1, 'scrollTo(0, 1000)');
  checkLastEntry(entries, 0,
                 [0, 100, 0, 100, 0, 100, 0, 100, 0,
                  document.documentElement.clientWidth, 0, document.documentElement.clientHeight,
                  true]);
}

function step1() {
  window.scrollTo(0, 0);
  checkLastEntry(entries, 0,
                 [0, 100, 0, 100, 0, 100, 0, 100, 0,
                  document.documentElement.clientWidth, 0, document.documentElement.clientHeight,
                  true]);
}
</script>