QMdiArea does not show the tab correctly when using QMdiArea::TabbedView and only adding one sub-window

Example to reproduce:

#include <QtGui>

int main(int argc, char **argv)
{
QApplication app(argc, argv);

QMdiArea mdiArea;
mdiArea.setViewMode(QMdiArea::TabbedView);
mdiArea.addSubWindow(new QPushButton("Hi I'm a button"));
mdiArea.show();

return app.exec();
}

However, adding two or more sub-windows works correctly. As a workaround one can call showMaximized() on the sub-window.