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/**
 * Get an element's distance from the top of the Document.
 * @private
 * @param  {Node} elem The element
 * @return {Number}    Distance from the top in pixels
 */
var getOffsetTop = function ( elem ) {
	var location = 0;
	if (elem.offsetParent) {
		do {
			location += elem.offsetTop;
			elem = elem.offsetParent;
		} while (elem);
	} else {
		location = elem.offsetTop;
	}
	location = location - headerHeight - settings.offset;
	return location >= 0 ? location : 0;
};