Qt: qabstractnativeeventfilter with qt::framelesswindowhint

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C++

Windows

QT

Hi,

I am using Qt for Windows.
I would like to have a window with a system menu and a dynamic Frameless flag. Without the Frameless flag, the window should have a system menu with a few additional entries; the user is free to toggle the Frameless flag on demand (e.g. by clicking a button), and the window should adjust.

I found a solution for the Windows system menu, using the Win32 API and a native event filter. The problem is that, when I install a native event filter, setting the Framless flag crashes the application (stack overflow).

I made a minimal example. Am I doing something wrong? Thank you for any hint.

Here is the code:

gui.h

#ifndef GUI_H
#define GUI_H

#include <QWidget>

namespace Ui {
class gui;
}

class gui : public QWidget
{
    Q_OBJECT

public:
    explicit gui(QWidget *parent = 0);
    ~gui();

private:
    //Ui::gui *ui;
};

#endif // GUI_H

gui.cpp

#include "gui.h"
//#include "ui_gui.h"

#include <qt_windows.h>
#include <QAbstractNativeEventFilter>
#include <QDebug>
#include <QMessageBox>
#include <QAbstractEventDispatcher>


#define MENUITEM_ABOUT 4324
gui *wnd = NULL;   // For the global EventFilter

class GlobalEventFilter : public QAbstractNativeEventFilter
{
    virtual bool nativeEventFilter(const QByteArray & /*eventType*/, void * message, long * /*result*/)
    {
        MSG *msg = (MSG*)message;

        if (wnd && msg->hwnd == (HWND) wnd->winId())
        {
            if(msg->message == WM_SYSCOMMAND)
            {
                if(msg->wParam == MENUITEM_ABOUT)
                {
                    qDebug() << "ABOUT";
                    //wnd->emitSignalAbout();
                    return true;
                }
            }
        }

        return false;
    }
} globalEventFilter;


gui::gui(QWidget *parent) :
    QWidget(parent)
//    ,ui(new Ui::gui)
{
    wnd = this;

    HMENU hMenu = ::GetSystemMenu((HWND) winId(), false);
    ::AppendMenu(hMenu, MF_SEPARATOR, 1, L"Separator");
    ::AppendMenu(hMenu, MF_STRING, MENUITEM_ABOUT, L"About");
    QAbstractEventDispatcher::instance(0)->installNativeEventFilter(&globalEventFilter);

    QAbstractEventDispatcher::instance(0)->removeNativeEventFilter(&globalEventFilter);
    setWindowFlag(Qt::FramelessWindowHint, true);
    QAbstractEventDispatcher::instance(0)->installNativeEventFilter(&globalEventFilter);

    //ui->setupUi(this);
}

gui::~gui()
{
    //delete ui;
}

What I have tried:

If I remove either the setWindowFlag, or the installNativeEventFilter calls, all works fine. With both of them, I get the stack overflow crash. If I move the setWindowFlag at the beginning of the function, it does not crash for some reason; if I toggle the flag later though, it crashes.