Vista style: Opening new windows with QSpinBox becomes increasingly slower

To reproduce, run attached example and keep pressing "launch another window". It gets some milliseconds slower with every new window.

Running in Luke Stackwalker shows that QFlat<enum Qt::WindowType>::operator& takes a lot of time, also some copy operations. They both seem to originate from QApplication::topLevelWidgets that is called by QWindowsXPStylePrivate::winId.