[REG 5 → 6] QQuickWidget causes dialog to be inadvertently maximized

Originally reported as https://bugs.kde.org/show_bug.cgi?id=478997

Minimal code example:

QMainWindow win;
auto button = new QPushButton("Push me baby one more time");
win.layout()->addWidget(button);

QObject::connect(button, &QPushButton::clicked, [&win]{
    QDialog *dlg = new QDialog(&win);
    QWidget *w = new QWidget(dlg);
    QQuickWidget *qw = new QQuickWidget(w);
    dlg->show();
});
win.show(); 

Full buildable code at https://invent.kde.org/nicolasfella/qquickwidgettest

When pressing the button when the main window is maximized the dialog will also be maximized (which it should not be). When the main window is not maximized the dialog will not be maximized either (as expected).

Removing the QQuickWidget makes it work as expected. Somehow the creation of the QQuickWidget influences the dialog's window state.

This works as expected in Qt 5.15.

Tested with Qt 6.6.1 and current 6.7 branch on Wayland and X11.