Steps to Reproduce

Sometimes, when selecting an item of a Combobox, a

java.lang.ClassCastException: org.zkoss.zul.Listcell cannot be cast to org.zkoss.zul.Comboitem
at org.zkoss.zul.Combobox.service(Combobox.java:990)
at org.zkoss.zk.ui.impl.DesktopImpl.service(DesktopImpl.java:826)
at org.zkoss.zk.ui.impl.UiEngineImpl.execUpdate(UiEngineImpl.java:1318)
at org.zkoss.zk.au.http.DHtmlUpdateServlet.process(DHtmlUpdateServlet.java:570)
at org.zkoss.zk.au.http.DHtmlUpdateServlet.doGet(DHtmlUpdateServlet.java:450)
at org.zkoss.zk.au.http.DHtmlUpdateServlet.doPost(DHtmlUpdateServlet.java:458)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:665)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:750)

is thrown. The exact reason is not clear to me, it is a bit random but happens from time to time. It might be good to check before casting to Comboitem, but the real reason is probably on the Javascript side. As a result of the exception, the item clicked is not selected.

Current Result

Expected Result

The item clicked should be selected, no error message should be displayed.

Debug Information