Scrollspy _getScrollTop() uses Window.scrollY which is not supported in IE

Created by: steve-32a

Current code: https://github.com/twbs/bootstrap/blob/v4-dev/js/src/scrollspy.js

_getScrollTop() {
  return this._scrollElement === window ?
      this._scrollElement.scrollY : this._scrollElement.scrollTop
}

MDN says the Window.scrollY property is not supported in IE. I was able to confirm this in IE (11.0.9600.18282) on Windows 7. https://developer.mozilla.org/en-US/docs/Web/API/Window/scrollY

One solution is to call jQuery.scrollTop() if scrollY is undefined. This works in IE on Windows 7. https://api.jquery.com/scrollTop/

_getScrollTop() {
  return this._scrollElement === window ?
      this._scrollElement.scrollY || $(this._scrollElement).scrollTop() : this._scrollElement.scrollTop
}