Bug fixed ZK-2231: Invisible window inside vbox cause js error when calculating flex size
http://tracker.zkoss.org/browse/ZK-2231
Bug fixed ZK-2200: Navigate combobox drop down list by "down" key not work correctly if contains empty string
http://tracker.zkoss.org/browse/ZK-2200
Merge remote-tracking branch 'Noah/B65-ZK-2231' into 6.5
Merge remote-tracking branch 'Noah/B65-ZK-2200' into 6.5