IntersectionObserver : Object

Spec:
https://www.w3.org/TR/intersection-observer/#intersectionobserver

----
new IntersectionObserver( \
    callback(entries : Array<IntersectionObserverEntry>, observer : IntersectionObserver) : undefined, \
    [options : { \
      root : Element, \
      rootMargins : String, \
      threshold : Number \
    }]) : IntersectionObserver

<htmlexample>
<ul></ul>
<script>
// Create an observer to see when item 15 intersects the viewport
const observer = new IntersectionObserver(entries => {
    entries.forEach(e => console.log(e.isIntersecting));
  });

const list = document.querySelector('ul');
for (let i = 0; i < 30; i++) {
  const listItem = document.createElement('li');
  listItem.textContent = 'item ' + i;
  list.appendChild(listItem);

  if (i === 15) {
    observer.observe(listItem);
  }
}
</script>
</htmlexample>

----
instance.root : Element

ReadOnly:
true

----
instance.rootMargin : String

ReadOnly:
true

----
instance.thresholds : Array<Number>

ReadOnly:
true

----
prototype.disconnect() : undefined

----
prototype.observe(target : Element) : undefined

----
prototype.takeRecords() : Array<IntersectionObserverEntry>

----
prototype.unobserve(target : Element) : undefined