Listbox's selected index becomes -1 when navigating with page up / page down key in CE

Steps to Reproduce

1. run selectedIndex.zul with ZK 7.0.3 CE
2. select an item in the listbox
3. press page down
4. press page up

Actual Result

the label blew the listbox shows -1

Expected Result

the label blew the listbox should show non-negative index number

Debug Information

this issue doesn't happen in EE.

Workaround

remove <custom-attributes org.zkoss.zul.listbox.rod="true" /> because listbox is in paging mold.