Element scrolled into view, with a 100 pixel threshold.

gistfile1.js

var checkActive = function(el) {

  var self = this,
    elRect = el.getBoundingClientRect(),
    winRect = document.documentElement.getBoundingClientRect(),
    isNearBottom = winRect.bottom - elRect.bottom < 100 && $(document).height() - ($(window).scrollTop() + $(window).height()) < 100;

  if (elRect.top < 100 && elRect.top > 0 || isNearBottom) {
    // Element is scrolled into view.
  }

  return self;
}