in js/popper-utils.js [242:255]
function getScroll(element) {
var side = arguments.length > 1 && arguments[1] !== undefined ? arguments[1] : 'top';
var upperSide = side === 'top' ? 'scrollTop' : 'scrollLeft';
var nodeName = element.nodeName;
if (nodeName === 'BODY' || nodeName === 'HTML') {
var html = element.ownerDocument.documentElement;
var scrollingElement = element.ownerDocument.scrollingElement || html;
return scrollingElement[upperSide];
}
return element[upperSide];
}