Check if element is fully in viewport

@param {object} el - popup instance DOM node
function (el) {
        var bounding = el.getBoundingClientRect();
        return (
            bounding.top >= 0 &&
            bounding.left >= 0 &&
            bounding.bottom <= (window.innerHeight || document.documentElement.clientHeight) &&
            bounding.right <= (window.innerWidth || document.documentElement.clientWidth)
        );
    }