change intersection threshold default to 0

[snewcomer]

To be inline with the IntersectionObserver spec, I think we should make the threshold value default to 0, meaning the didEnterViewport will trigger when the first pixel of the target has entered the scrollable area.

[snewcomer]

Also, there may be one big reason for this (that is outside of infinite scrolling). isIntersecting return true, even when the element is scrolled outside of the viewport (if threshold is anything > 0). When threshold is configured to 0/not configured at all, isIntersecting is false when it fails the threshold value as it is scrolled out of the viewport. This is due to why isIntersecting was created - see links below. Gist of it is zero-area elements have an intersectionRatio of 1 by default so can be used to detect isIntersecting.

Turns out this is how it was supposed to be intended.

w3c/IntersectionObserver#165
https://bugs.chromium.org/p/chromium/issues/detail?id=713819