// this is the child QMainWindow
if (this->m_infoAreaFloating)
{
    // this should give me a floating window besides the main window
    this->setWindowFlags(Qt::Desktop);
    this->show();
}
else
{
    // make this compliant as QWidget
    this->setWindowFlags(this->windowFlags() & ~Qt::Window);
}

setWindowFlags(m_infoAreaFloating ? Qt::Window : Qt::Widget);
show();