steps to reproduce

1. specify the property below in zk.xml

   <library-property>
          <name>org.zkoss.zul.listbox.checkmarkDeselectOthers</name>
          <value>true</value>
   </library-property>
   

2. run the zul below

   <zk>
   	<listbox width="250px" height="100px" multiple="true" checkmark="true" mold="paging" pageSize="2"
                onSelect="count.setValue(self.getSelectedCount());" onPaging="count.setValue(self.getSelectedCount());" >
          <listitem label="hello" />
          <listitem label="world" />
          <listitem label="hello" />
          <listitem label="you" />
   </listbox>
   <intbox id="count"  />
   </zk>
   

3. click the 1st and 2nd item's checkmark (insteaf of a listitem itself) , selection count is 2
4. switch to the next page click the 1st item's checkmark
5. de-select the 1st item by clicking its checkmark

result

selection count is 0
because Listbox clears the previous selection.

expected result

selection count is 2