Fix #12610, remove unneeded window.event. Close gh-968.

src/event.js

@@ -355,7 +355,7 @@ jQuery.event = {
 	dispatch: function( event ) {
 
 		// Make a writable jQuery.Event from the native event object
-		event = jQuery.event.fix( event || window.event );
+		event = jQuery.event.fix( event );
 
 		var i, j, cur, ret, selMatch, matched, matches, handleObj, sel,
 			handlers = ( (jQuery._data( this, "events" ) || {} )[ event.type ] || []),