QComboBox items fall through the window

#include <QApplication>
#include <QWidget>
#include <QComboBox>
#include <QBoxLayout>

int main(int argc, char *argv[])
{
 QApplication a(argc, argv);

QWidget w;
 QVBoxLayout l( &w );
 QComboBox cmb( &w );
 cmb.addItems( QStringList() << "First" << "Second" << "Third" << "Fourth" );
 l.addWidget( &cmb );

// without Qt::WindowStaysOnTopHint it works correctly
 w.setWindowFlags( Qt::WindowStaysOnTopHint );
 w.show();

return a.exec();
}