Impossible to create a QDockWidget which always floats on top of its parent

It is not possible to create an undockable QDockWidget which is always on top of the parent. When calling

...
QDockWidget * dock = new QDockWidget("test dock", &mainWin);
dock->setAllowedAreas (Qt::NoDockWidgetArea);
dock->setFloating(true);
dock->show();
...

then double clicking on the widget's bar will cause the QDockWidget to be docked.