Core: Handle IE11 bug with document.activeElement in iframes

Fixes #11539
Closes gh-1518

(cherry picked from commit 76c27556f4)

ui/core.js

@@ -57,6 +57,13 @@ $.extend( $.ui, {
 			activeElement = document.body;
 		}
 
+		// Support: IE 11 only
+		// IE11 returns a seemingly empty object in some cases when accessing
+		// document.activeElement from an <iframe>
+		if ( !activeElement.nodeName ) {
+			activeElement = document.body;
+		}
+
 		return activeElement;
 	},