Similar scenario to
WICKET-5378 - Create a ModalWindow, add an AutoCompleteTextField. Use IE7 and open the ModalWindow. Enter text in the AutoCompleteTextField, and depending on the size of your ModalWindow you might not see anything, because the dropdown suggestions list appears behind the ModalWindow (the z-index is wrong).
Although this bug is present in 6.11.0, my quickstart uses 6.9.1 because it is easier to see the bug there as
WICKET-5378 obscures it in 6.11.0 by moving the dropdown suggestions far away from the ModalWindow. But if you want to see it in 6.11.0, change the wicket.version in the POM and resize the ModalWindow to be larger (so that it at least partically covers the area where the dropdown suggestions list appears).