add pixels to qgraphicsitem

[MarekR22]

From my experience you should restore state of painter at the end of paint.
Without it you can have strange artifacts. Of course if you are using some optimization flag to auto store painter then this problem will not appear.
When you are writing own graphics item you shouldn't try to evaluate impact of transformation, especially when you have little experience with graphics freamework.

Try correct this code like that:

class MyRect : public QObject, public QGraphicsRectItem
{
    Q_OBJECT
 
public:
    MyRect( QGraphicsItem *pParent=0 )
     :  QGraphicsRectItem( pParent )
    {
        setItUp();
    }
    MyRect( const QRectF & rect, QGraphicsItem* pParent=0 ) 
     :  QGraphicsRectItem( rect, pParent )
    {
        setItUp();
    }
    ~MyRect() {}
 
    void setItUp()
    {
        m_penWidth = 1.0;
        setPen( QPen(Qt::green) );
        setAcceptHoverEvents( true );
        setFlag( QGraphicsItem::ItemIsSelectable, true );
    }
 
    QRectF boundingRect() const
    { //looks ok
        return this->rect().adjusted( -m_penWidth, -m_penWidth,
                                       m_penWidth,  m_penWidth );
    }
 
protected:
    void paint( QPainter *painter,
                const QStyleOptionGraphicsItem *option,
                QWidget *widget )
    {
        Q_UNUSED(widget);
 
        // this way is faster then painter->store();
        QPen oldPen = painter->pen();
        QBrush oldoldBrush = painter->brush();
 
        painter->setBrush(brush());
        if (option->state & QStyle::State_Selected) {
            painter->setPen(pen());
        } else {
            QPen hilitePen(QBrush(Qt::red),m_penWidth);
            painter->setPen(hilitePen);
        }
        painter->drawRect(rect());
 
        // restore painter:
        painter->setPen(oldPen);
        painter->setBrush(oldBrush);
    }
 
private:
    qreal m_penWidth;
};

class MyRect : public QObject, public QGraphicsRectItem
{
    Q_OBJECT

public:
    MyRect( QGraphicsItem *pParent=0 )
     :  QGraphicsRectItem( pParent )
    {
        setItUp();
    }
    MyRect( const QRectF & rect, QGraphicsItem* pParent=0 ) 
     :  QGraphicsRectItem( rect, pParent )
    {
        setItUp();
    }
    ~MyRect() {}

    void setItUp()
    {
        m_penWidth = 1.0;
        setPen( QPen(Qt::green) );
        setAcceptHoverEvents( true );
        setFlag( QGraphicsItem::ItemIsSelectable, true );
    }

    QRectF boundingRect() const
    { //looks ok
        return this->rect().adjusted( -m_penWidth, -m_penWidth,
                                       m_penWidth,  m_penWidth );
    }

protected:
    void paint( QPainter *painter,
                const QStyleOptionGraphicsItem *option,
                QWidget *widget )
    {
        Q_UNUSED(widget);

        // this way is faster then painter->store();
        QPen oldPen = painter->pen();
        QBrush oldoldBrush = painter->brush();

        painter->setBrush(brush());
        if (option->state & QStyle::State_Selected) {
            painter->setPen(pen());
        } else {
            QPen hilitePen(QBrush(Qt::red),m_penWidth);
            painter->setPen(hilitePen);
        }
        painter->drawRect(rect());

        // restore painter:
        painter->setPen(oldPen);
        painter->setBrush(oldBrush);
    }

private:
    qreal m_penWidth;
};

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Another thing why you are trying do it yourself?
Why not just manipulate properties of QGraphicsRectItem?
Just use QAbstractGraphicsShapeItem::setBrush and QAbstractGraphicsShapeItem::setPen when state of item is changed.
Re-implement itemChange method to react on that.