Since 1.4.16 the datepicker is broken in ModalWindow : it is display under the modal when the modal is small.
Before 1.4.16 the datepicker was display above the modalwindow.
See screenshot :

This seems to be the consequence of WICKET-2056.
The folowing CSS has been addded in modal.css :
div.wicket-modal div.w_content_container {
position: relative;
}
Shall I create a new jira?