There was this change from 4.7.1 to 4.7.2:
In function qcombobox.cpp / QComboBoxPrivateContainer::leaveEvent

view->clearSelection(); // 4.7.1

has been changed for:

view->setCurrentIndex(QModelIndex()); // 4.7.2

(this is inside an #ifdef Q_WS_MAC section)

but if leaveEvent is called before QEvent::MouseButtonRelease (which calls emit itemSelected(view->currentIndex()))
then nothing is selected as currentIndex() has been made unvalid.

This is what happens here, always, hence combo boxes are unusable. Curiously, this only happens with the debug libs.
Reverting the change to what was in 4.7.1, solves the problem.