Smooth the corners of shaped widget

[al072]

Hi dear Qt community! I need some help with shaped widget corner smooth rendering. I create shaped widget based on QPolygon using setMask function this way (QT 5.15.2):

Widget::Widget(QWidget *parent)
    : QWidget(parent,Qt::FramelessWindowHint | Qt::WindowSystemMenuHint)
    , ui(new Ui::Widget)
{
    QPainterPath path;
    path.addPolygon(poly);
    QRegion region(path.toFillPolygon().toPolygon());

    this->setMask(region);
    ui->setupUi(this);
}

Widget looks like on the screen (attached), please take a look on the left top corner it is not smoothed:

Could somebody give me a hint please how to make the polygon corners smooth? Thank you very much., i am new to Qt sorry if the question is not correct.