[REG 5.5.1->5.6 RC] iOS: When using QOpenGLWidget, any other widgets are displayed as black

#include <QApplication>
#include <QMainWindow>
#include <QOpenGLWidget>
#include <QPushButton>

class GLWidget : public QOpenGLWidget
{
public:
    explicit GLWidget(QWidget *parent = 0) : QOpenGLWidget(parent)
    {
    }

private:
    void initializeGL()
    {
        glClearColor(0.3f, 0.6f, 0.9f, 1.f);
    }

    void resizeGL(int width, int height)
    {
        glViewport(0, 0, width, height);
    }

    void paintGL()
    {
        glClear(GL_COLOR_BUFFER_BIT);
    }
};


class MainWindow : public QMainWindow
{
public:
    explicit MainWindow(QWidget *parent = 0) : QMainWindow(parent)
    {
        glWidget_ = new GLWidget(this);
        button_ = new QPushButton("button", this);
    }

private:
    void resizeEvent(QResizeEvent *)
    {
        glWidget_->setGeometry(0, 0, width(), height() - 100);
        button_->setGeometry(0, height() - 100, width(), 100);
    }

private:
    GLWidget *glWidget_;
    QPushButton *button_;
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();

    return a.exec();
}