If multiple selection is to be hold across a listbox' pages the user must keep ctrl- pressed down for the checkmarks don't behave as expected: they get checked if the user clicks anywhere on the listitem, but if they are not clicked on directly, clearFirst() is signaled when changing the page.
More details here http://www.zkoss.org/forum/listComment/14800 and here http://www.zkoss.org/forum/listComment/16592
Fixed since 2011/7/13.