I came across a problem regarding tabs which are backed-up by a model.
I wrote a test case : http://zkfiddle.org/sample/3220424/2-Tabs-with-Model
NB: due to bug ZK-2973, you may not want to run this example in Firefox.
There is a model for the tabs, which is also used by the listbox. The tabs or the list items can be re-ordered by drag and drop.
If you drop a tab on the last one, the dropped tab will correctly be moved to the end of the list. But if you drop it anywhere else, the tabs will be all messed up.
The list doesn't seem to have the same problem, you can drag and drop the elements in any place.