Function isScrolledPast

src/utils.js:284–307

* Checks if a side of an element is scrolled past a side of its parents * @param {HTMLElement} el The element who's side being scrolled out of view is in question * @param {String} elSide Side of the element in question ('top', 'left', 'right', 'bottom') * @param {String

(el, elSide, parentSide)

 * @return {HTMLElement}               The parent scroll element that the el's side is scrolled past, or null if there is no such element
 */
function isScrolledPast(el, elSide, parentSide) {
	let parent = getParentAutoScrollElement(el, true),
		elSideVal = getRect(el)[elSide];

	/* jshint boss:true */
	while (parent) {
		let parentSideVal = getRect(parent)[parentSide],
			visible;

		if (parentSide === 'top' || parentSide === 'left') {
			visible = elSideVal >= parentSideVal;
		} else {
			visible = elSideVal <= parentSideVal;
		}

		if (!visible) return parent;

		if (parent === getWindowScrollingElement()) break;

		parent = getParentAutoScrollElement(parent, false);
	}

	return false;
}