Height is not really fixed with small heights. This cause odd behavior for example small floating toolbars with frames. QWidget is base class for other widget so also dockwidgets have the same issue. Bug prevents defining small floating widgets with fixed size.

Not happening on Mac OS or Qt 4.8. Assumption is that this is Windows QPA specific issue.

Test code. Just try to resize window height to see effect. Should be fixed height.

#include <QApplication>
#include <QWidget>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    QWidget w;
    w.setGeometry(500,500,200,50);
    w.setFixedHeight(50);
    w.show();
    return a.exec();
}