title: Intersection observer thresholds value not working
body: <p>Thresholds option in Intersection Observer doesn't seem to work.</p>
  
  <p>To give more context, my list items are rendered in viewport and I want each item to observe on viewport's Intersection observer. The thing is, I need to observe them in steps of 25%. Both when they get in viewport and/or go out of viewport.</p>
  
  <p>As per documentation it seems that providing thresholds array as <code>[0, 0.25, 0.50. 0.75, 1]</code> should do the work, but actually it doesn't make any difference than giving it a value of 0 or 1.
  I want to observe changes everytime an element's intersection increases or decreases by 25% in my scroll view.</p>
  
  <p>You can check my code on - <a href="https://codesandbox.io/s/rm46m1qy7o" rel="nofollow noreferrer">https://codesandbox.io/s/rm46m1qy7o</a></p>
  
  <p>I am using chrome v72 on Windows 10.</p>