Window event behavior has changed
In order to provide more consistency with industry standards and other popular browsers, behavior of window.event has changed in Internet Explorer 11. For webpages (and apps) displayed in IE11 edge mode:
The object passed to the handler of a window.event property returns "undefined" when the event does not have a handler assigned. (In earlier document modes, it returned NULL in this case.)
Update any webpages affected by this behavior so they don't depend on legacy functionality.