function _elementInViewport(el) {
    var rect = el.getBoundingClientRect();

    return (
      rect.top >= 0 &&
      rect.left >= 0 &&
      (rect.bottom+80) <= window.innerHeight && // add 80 to get the text right
      rect.right <= window.innerWidth
    );
  }
Check to see if the element is in the viewport or not
http://stackoverflow.com/questions/123999/how-to-tell-if-a-dom-element-is-visible-in-the-current-viewport

@api private
@method _elementInViewport
@param {Object} el