QStyleOptionDockWidget does a cast to QDockWidget

On Windows, you are unable to draw a custom dock widget title, because the windows style does a cast to a QDockWidget first:
(Line 1428 of qwindowsvistastyle.cpp)

    case CE_DockWidgetTitle:
        if (const QDockWidget *dockWidget = qobject_cast<const QDockWidget *>(widget))

As it only does a cast to check if the widget is floating, it would make more sense to add "floating" as a boolean in the style option.