Change the window flag to the Widget from the Window at the focusOutEvent() will crash

void Window::focusOutEvent( QFocusEvent * event )
{
    setWindowFlags( Qt::Widget ); // Crash when pressing tab key under full screen
    //showNormal();
}
void Window::keyPressEvent(QKeyEvent *e)
{
    if (e->key() == Qt::Key_0)
    {
        setWindowFlags( Qt::Window );
        showFullScreen();
        setFocus();
    }
    else
        QWidget::keyPressEvent(e);
}