Although onAnchorPos event would not fire if the anchor position stayed the same as the last time it fired, multiple onAnchorPos events were fired when anchor top alternates between an integer value (638) and a fractional value (638.400...). This behavior causes the number of request parameter to exceed the maximum value of 100. Please see the server log posted on http://forum.zkoss.org/question/92611/illegalstateexception-too-many-parameters-error/.
The first few entries of the parameter list read as follows.
1. Select a listitem
2. Scroll down until rod kicked in
3. Scroll up and down rapidly while rod is still loading data
4. Click a few times at random position inside listbox while rod is still loading data
5. Observe in network tab of browser's developer tool
-> look for large post body and it would contain many onAnchorPos events with similar anchor positions