floating QDockWidget corruption

QDockwidget's titlebar buttons and some content is not displayed when undocking the widget (making it float).
For example, create a simple mainwindow with a QDockWidget containing an QGLWidget. Try to undock it -> bug
Same problem happens when the QDockWidget 's widget (that is set through setWidget()) has the mainwindow as parent.

Sample code to reproduce the error:
When trying to make a dock widget float, and title bar buttons will disappear
-------------------------------------------------------------------------------------------------------------

#include <QApplication>
#include <QPushButton>
#include <QDockWidget>
#include <QMainWindow>
#include <QLabel>
#include <QtOpenGL/QGLWidget>

int main(int argc, char *argv[])
{
QApplication app(argc, argv);
QMainWindow w;

QPushButton hello("Hello world!");
hello.resize(100, 30);
w.setCentralWidget(&hello);
QDockWidget *dock = new QDockWidget("gl");
dock->setAllowedAreas(Qt::TopDockWidgetArea);
dock->setWidget(new QGLWidget());
w.addDockWidget(Qt::TopDockWidgetArea, dock);

QDockWidget *dock2 = new QDockWidget("hi");
dock2->setAllowedAreas(Qt::RightDockWidgetArea);
dock2->setWidget(new QLabel("hello", &w));
w.addDockWidget(Qt::RightDockWidgetArea, dock2);

w.show();
return app.exec();
}