fit-content-block-size-fixedpos-ref.html

<!DOCTYPE html>
<style>
#container {
  height: 90vh;
  position: fixed;
  top: 50%;
  background-color: gold;
  margin: auto;
  bottom: 0;
}
</style>
<div id="container">
  <div style="height:90vh;width:50px;outline: 2px dotted red;"></div>
</div>

chromium/third_party/blink/web_tests/external/wpt/css/css-sizing/fit-content-block-size-fixedpos-ref.html