title: IntersectionObserver isIntersecting property never is false if threshold greater than 0
body: <p>If I configure the <code>threshold</code> value to anything greater than 0, <code>isIntersecting</code> never returns false when the target leaves the viewport.  However, if I leave the default threshold at 0, <code>isIntersecting</code> will return false when the target exits the viewport.  See example below and delete <code>threshold: 1.0</code>.</p>
  
  <p><a href="https://jsfiddle.net/snewcomer24/get0a4xr/1/" rel="noreferrer">https://jsfiddle.net/snewcomer24/get0a4xr/1/</a></p>
  
  <p>This seems like divergent behavior.  Does anybody have an explanation for this behavior?</p>