When you set QGLWidget as central widget to QMainWindow, and run this window in fullscreen, menu bar is not visible. It however reacts to user input, when you click where active menu is, it expands and you see submenu. We find out sometimes helps when you call setVisible(true) before calling showFullScreen(), unfortunately in some our apps this call hangs (setVisible) so it's not legit workaround for this bug. Also we tested it in QT 4.x, and it works, menu is visible.

main.cpp
>>>>>>>>>>>>>>>>>>>>>>>>>>>>
#include "MainWin.h"
#include <QApplication>
#include <QtGui/QtGui>

int main(int argc, char *argv[])
{
QApplication app(argc, argv);
MainWin window;
//window.setVisible(true); // uncomment to make menu in fullscreen visible
window.showFullScreen();
return app.exec();
}
//////////////////////////////////////
MainWin.h
//////////////////////////////////////
#pragma once

#include <QtOpenGL/QtOpenGL>
#include <QtGui/QtGui>

class MainWin : public QMainWindow
{
Q_OBJECT

public:
MainWin(QWidget* parent = 0)
:QMainWindow(parent)

{ menu = menuBar()->addMenu(tr("&File")); menu->addAction(tr("&Quit"), this, SLOT(close())); setCentralWidget(new QGLWidget()); }

protected:
QMenu* menu;
};