intersectionRatio === 0 is not a safe assumption for "exit"

[nolanlawson]

This code may lead to very subtle bugs because, per the spec, an element may still be "intersecting" even though intersectionRatio is 0. For instance, this situation is possible:

{
  isIntersecting: true,
  intersectionRatio: 0
}

This is due to how edge adjacency works, and you can read the gory details here (w3c/IntersectionObserver#222). You can repro this by scrolling such that the element perfectly overlaps the root (may be easiest to repro in Firefox currently).

My recommendation would be to change the if() statement to

if (typeof entry.isIntersecting === 'boolean' ? !entry.isIntersecting : entry.intersectionRatio === 0) {
  onExit()
}

The case where typeof entry.isIntersecting isn't a boolean covers Edge 15, which does not support isIntersecting, and where entry.intersectionRatio === 0 is actually a safe proxy for "no longer intersecting."

[yoshuawuyts]

Thanks heaps! Reworked the API, and included this lil snip. Should be a lot better now! ✨

📦 v2.0.0