If the C++ API is used to make a multi-select list box using some of these constructs included here just for orientation

Reference<XInterface> model = xMSF->createInstance(		OUSTRING("com.sun.star.awt.UnoControlListBoxModel"));
...
PropertySetHelper(model).set("MultiSelection", Any(multiSelection))
...
Reference<XListBox> result(insertByName(name, model), UNO_QUERY);

and an item selection listener is configured

Reference<XItemListener> xItemListener = static_cast<XItemListener*>(&listener);
...
result->addItemListener(xItemListener);

then the listener will be called both when a selection is made and also when a second selection is made or the first is unmade by, for example, clicking on a previously selected item with the control key pressed.  When nothing is selected (or maybe more than one item is selected) the usual procedure is to indicate that item -1 is selected.  Instead, item 65535 is selected.

Somewhere in the OpenOffice code a sal_Int16 value of -1 is being converted to a sal_Int32 value in the wrong way.  As a result, the listener's code needs a work-around similar to this:

void onListSelected(int id, sal_Int32 selected) {
	bool hide = selected < 0 || selected == 65535;
...

It would be good to track that down and fix it.

This behavior has been observed for both Windows 7 and Fedora Linux64, but I think the problem is probably more general.