Listbox.doubleclick

I experience weird behaviour: Mac OSX10.8, XOJO 3, Desktop
there are no events for the window
I have two ways to load a listbox from the DB. (The user can come from different directions)
a) the method is called from the previous window with the search criteria as the parameter
b) the window is opened and the user enters the search parameter
I load the listbox with the same method
When I double click,
a) brings the expected results (listindex as selected)
b) always selects the first row (listindex as -1)
Has anyone got any clue what might cause this inconsistency?