title: position:fixed doesn't work inside cdk-virtual-scroll-viewport
body: <p>HTML element with <strong>position:fixed</strong> doesn't work inside <strong>cdk-virtual-scroll-viewport</strong>.
  Below style should put the element at the page top left corner, but this class doesn't apply to elements inside cdk-virtual-scroll-viewport.</p>
  <p>.fixed-div {position: fixed;top: 0;left: 0;}</p>
  <p>for some reason above class to elements inside cdk-virtual-scroll-viewport will keep the elements at the top left corner of cdk-virtual-scroll-viewport.</p>
  <p>link to reproduce <a href="https://stackblitz.com/edit/angular-mvf6dp" rel="nofollow noreferrer">https://stackblitz.com/edit/angular-mvf6dp</a></p>
  <p>How to force elements with position:fixed inside cdk-virtual-scroll-viewport to display the relative to viewport ?</p>