[macOS] Foreign window support not working

#include <QtGui>
#include <QtWidgets>

// Mimic KwindowSystem::setMainWindow
void KWindowSystemSetMainWindow(QWidget *subWidget, WId mainWindowId) {
    // Set the WA_NativeWindow attribute to force the creation of the QWindow.
    // Without this QWidget::windowHandle() returns 0.
    subWidget->setAttribute(Qt::WA_NativeWindow, true);
    QWindow *subWindow = subWidget->windowHandle();
    Q_ASSERT(subWindow);

    QWindow *mainWindow = QWindow::fromWinId(mainWindowId);
    if (!mainWindow) {
        // foreign windows not supported on all platforms
        return;
    }

    // mainWindow is not the child of any object, so make sure it gets deleted at some point
    QObject::connect(subWidget, &QObject::destroyed, mainWindow, &QObject::deleteLater);
    subWindow->setTransientParent(mainWindow);
}

class D : public QDialog {
public:
    explicit D(QWidget *parent = nullptr)
        : QDialog(parent)
    {
        auto closeButton = new QPushButton(QString::fromLatin1("Close"));
        connect(closeButton, &QPushButton::clicked, this, &QDialog::accept);

        auto vLayout = new QVBoxLayout(this);

        vLayout->addWidget(closeButton);
    }
};

class W : public QMainWindow {
public:
    explicit W(QWidget *parent = nullptr)
        : QMainWindow(parent)
    {
        auto autoDelete = new QCheckBox(QString::fromLatin1("Dlg DeleteOnClose"));
        auto usePointer = new QCheckBox(QString::fromLatin1("Dlg use pointer"));

        auto inProcess = new QPushButton(QString::fromLatin1("In-Process"));
        connect(inProcess, &QPushButton::clicked, this, [this, autoDelete, usePointer]() {
            process(winId(), autoDelete->isChecked(), usePointer->isChecked());
        });

        auto outProcess = new QPushButton(QString::fromLatin1("Out-Process"));
        connect(outProcess, &QPushButton::clicked, this, [this, autoDelete, usePointer]() {
            const QString cmd = QApplication::applicationFilePath();
            const QStringList args = { QString::number(winId()),
                                       QString::number(autoDelete->isChecked() ? 1 : 0),
                                       QString::number(usePointer->isChecked() ? 1 : 0) };
            QProcess::startDetached(cmd, args);
        });

        auto centralWidget = new QWidget(this);
        auto vLayout = new QVBoxLayout(centralWidget);

        vLayout->addWidget(autoDelete);
        vLayout->addWidget(usePointer);
        vLayout->addWidget(inProcess);
        vLayout->addWidget(outProcess);

        setCentralWidget(centralWidget);
    }

    void process(WId id, bool autoDelete, bool usePointer) {
        qWarning() << Q_FUNC_INFO << id << autoDelete << usePointer;

        const auto setup = [id, autoDelete](QDialog *dlg) {
            dlg->setAttribute(Qt::WA_DeleteOnClose, autoDelete);
            KWindowSystemSetMainWindow(dlg, id);
            dlg->exec();
        };

        if (usePointer) {
            auto d = new D;
            setup(d);
        } else {
            D d;
            setup(&d);
        }
    }
};

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    QApplication::setApplicationName(QFileInfo(QString::fromLatin1(argv[0])).baseName());
    QApplication::setApplicationDisplayName(QApplication::applicationName());

    const WId id = argc > 1
            ? static_cast<quintptr>(QString::fromLatin1(argv[1]).toULongLong())
            : 0;
    const bool deleteOnClose = argc > 2
            ? QString::fromLatin1(argv[2]).toInt() == 1
            : 0;
    const bool usePointer = argc > 3
            ? QString::fromLatin1(argv[3]).toInt() == 1
            : 0;

    W w;
    w.setWindowTitle(QApplication::applicationDisplayName());

    if (id != 0) {
        w.process(id, deleteOnClose, usePointer);
    } else {
        w.show();
    }

    return app.exec();
}