Widget can not show correctly if there's a fullscreen QOpenGLWidget

A minimal test code:

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

int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QOpenGLWidget w;
QObject::connect(new QPushButton("show label", &w), &QPushButton::clicked, []()

);
w.showFullScreen();
return a.exec();
}

"new button" should be in front of w. But the result is behind w. If change w to QWidget, the button shows correctly.