QGroupBox title looks enabled while group box is disabled

#include <QLabel>
#include <QGroupBox>
#include <QWidget>
#include <QVBoxLayout>
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication app(argc,argv);
    QLabel *label = new QLabel("label");
    QGroupBox *groupbox1 = new QGroupBox("group box 1");
    QGroupBox *groupbox2 = new QGroupBox("group box 2");
    groupbox1->setCheckable(false);
    groupbox2->setCheckable(true);
    QVBoxLayout *layout = new QVBoxLayout();
    layout->addWidget(label);
    layout->addWidget(groupbox1);
    layout->addWidget(groupbox2);
    QWidget widget;
    widget.setLayout(layout);
    widget.setEnabled(false);
    widget.show();
    return QApplication::exec();
}