_getScrollParent(element) {
    const {overflowY} = window.getComputedStyle(element);
    const isScrollable = overflowY !== 'visible' && overflowY !== 'hidden';

    if (isScrollable) {
      return element;
    }

    if (element.parentElement) {
      return this._getScrollParent(element.parentElement);
    }

    return document;
  }
Finds the first scrollable ancestor of `element`. Falls back to the document.
@param {Element} element
@return {Element | Document}