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chromium/third_party/blink/web_tests/fast/block/positioning/fixed-in-abs-height-change.html 

<!DOCTYPE html>
<script src="../../../resources/testharness.js"></script>
<script src="../../../resources/testharnessreport.js"></script>
<style>

.fixed {
  left: 100px;
  width: 600px;
  height: 200px;
  position: fixed;
}
.box {
  position: absolute;
  bottom: 0;
  width: 600px;
  height: 100px;
  background: lightblue;
}

</style>
<div style="position: absolute">
  <div class="fixed">
    <div class="box"></div>
  </div>
</div>
<script>

test(() => {
  var fixed = document.querySelector(".fixed");
  var box = document.querySelector(".box");

  assert_equals(box.offsetTop, 100);

  fixed.style.height = "400px";
  assert_equals(box.offsetTop, 300);
}, "Fixed-pos in abs-pos container updates children on height change.");

</script>