if Qt::AA_NativeWindows is set on the application QDockeWidget does not behaves as expected. Resizing of docked widget does not work. When it is undocked it can be resized.

This can be easily seen with Qt's Example examples/widgets/mainwindows/dockwidgets

QApplication app(argc, argv);
app.setAttribute(Qt::AA_NativeWindows); // add this line
Q_INIT_RESOURCE(dockwidgets);