title: offsetTop / getBoundingClientRect().top of the element with position: sticky
body: <p>When element with <code>position: sticky</code> reach his offset (<code>top: 0</code>, for example) and stops on the screen, i can't get his <code>offsetTop</code> properly, it shows more than necessary. At the same time, I can't get <code>getBoundingClientRect().top</code>, because it shows <code>0</code>. Please take a look at this example:</p>
  
  <p><a href="https://jsfiddle.net/Lxud76ma/" rel="nofollow noreferrer">https://jsfiddle.net/Lxud76ma/</a></p>
  
  <p>When the element is sticky, it reserves its place in DOM, but the calculations are takes on its actual location.</p>
  
  <p>So, how can I get coordinates of an element when it is sticky? Any ideas?</p>