Unadjusted bouding box with setCosmetic(true)

I want to display "basic" segments in a QGraphicsView with fixed width for the segments, zoom in/out feature and efficient bouding boxes.

The zoom is implemented that way:

Code:

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QMatrix vMatrix; 
vMatrix.scale(HorizontalScale(), VerticalScale());
setMatrix(vMatrix);
scene()->update();

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And the segment that way:

Code:

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class Segment : public QGraphicsView {
 
    qreal mX1;
    qreal mY1;
    qreal mX2;
    qreal mY2;
 
    void paint(QPainter* aPainter, const QStyleOptionGraphicsItem* aOption, QWidget* aWidget) {
        QPen vPen(QColor(0, 0, 255));
        vPen.setCosmetic(true);
        vPen.setWidth(3);    
 
        QPaintDevice *vDevice = aPainter->device();
        aPainter->setPen(vPen);
        aPainter->drawLine(mX1, mY1, mX2, mY2);
    }
 
    QRectF boundingRect() const {
        qreal vHalfWidth = sPen.width() / 2.0f;
        qreal vX = qMin(mX1, mX2) - vHalfWidth; 
        qreal vY = qMin(mY1, mY2) - vHalfWidth; 
        qreal vWidth = qAbs(mX2 - mX1) + vHalfWidth*2; 
        qreal vHeight = qAbs(mY2 - mY1) + vHalfWidth*2; 
 
        return QRectF(vX, vY, vWidth, vHeight);
    }
}

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With the code above and an horizontal segment. The bouding box height is always returned to 3. But when zoomin, the segment always has its 3 pixels width (thanks to setCosmetic(true)) BUT the bounding box is stretched larger than the segment drawing.

Any idea on how to handle this ?