Datepicker: Now uses zIndex() to determine zIndex for dpDiv on show (Fixed #4580 Datepicker appears underneath other widgets)

Author: btburnett3

ui/jquery.ui.datepicker.js

@@ -642,6 +642,7 @@ $.extend(Datepicker.prototype, {
 					css({left: -borders[0], top: -borders[1],
 						width: inst.dpDiv.outerWidth(), height: inst.dpDiv.outerHeight()});
 			};
+			inst.dpDiv.zIndex($(input).zIndex()+1);
 			if ($.effects && $.effects[showAnim])
 				inst.dpDiv.show(showAnim, $.datepicker._get(inst, 'showOptions'), duration, postProcess);
 			else