The mask does not work with QRect(0, 0, ui->widget->width(), ui->widget->height().

    QBitmap windowMask(ui->widget->width(), ui->widget->height());
    windowMask.fill(Qt::transparent);
    QPainter p(&windowMask);
    // 
    p.fillRect(QRectF(0, 0, ui->widget->width(), ui->widget->height()), Qt::white);
    p.end();
    ui->widget->setMask(windowMask);

But it work fine with 1px more
p.fillRect(QRectF(1, 1, ui->widget->width(), ui->widget->height()), Qt::white);

How do I cover the entire widget?