When changing item in QGraphicsScene, the old content is still displayed

QtUser467

I have a QGraphicsScene that contains some QGraphicsItem. One is a custom item derived from QGraphicsItem, something like so (simplified):

class MyPath : public QGraphicsItem {
  public:
    enum { Type = UserType + 2 };

  public:
    MyPath();
    virtual int type() const override { return Type; }
    virtual QRectF boundingRect() const override;

    void start(const QPointF& p, double w);
    void stop(const QPointF& p, double w);
    void line_to(const QPointF& p, double w);

    virtual void paint(QPainter* painter,
                       const QStyleOptionGraphicsItem* option,
                       QWidget* widget = nullptr) override;

  private:
    QPen _pen;
    Path _xy_path;
    QPainterPath _path;
};

The implementation is like this:

void MyPath::start(const QPointF& p) {
    prepareGeometryChange();
    _path.moveTo(p);
    _xy_path.append(p);
}

void MyPath::stop(const QPointF& p) {
    prepareGeometryChange();
    _path.lineTo(p);
    _xy_path.append(p);

    // simplify the path
    _xy_path.douglas_peucker(5);

    prepareGeometryChange();
    _path.clear();

    _path.moveTo(_xy_path[0]);
    const auto size = _xy_path.size();
    for (int i = 0; i < size; i++) _path.lineTo(_xy_path[i]);
}

void MyPath::line_to(const QPointF& p) {
    prepareGeometryChange();
    _path.lineTo(p);
    _xy_path.append(p);
}

QRectF MyPath::boundingRect() const {
    double delta = _pen.width() / 2.0 + 5;
    QRectF rect = _path.boundingRect();
    rect.adjust(-delta, -delta, delta, delta);
    return rect;
}

void MyPath::paint(QPainter* painter,
                       const QStyleOptionGraphicsItem* option,
                       QWidget* widget) {
    painter->save();
    painter->setPen(_pen);
    painter->drawPath(_path);
    painter->restore();
}

The problem now is that the old drawing remains on the screen even when the old path is erased. The new, simplified path is just drawn on top. Here is an example:
the straight line segments should be there, but the hand-drawn line should have disappeared.

qgraphicsitem.png.png

It seems like prepareGeometryChange() is not sufficient to make the old line disappear, which I would have assumed. How do I do this properly?

QtUser467

Upon further inspection, it turned out that the problem only occurs when the widget containing the QGraphicsView has the attribute Qt::WA_TranslucentBackground set.

setAttribute(Qt::WA_TranslucentBackground);

Is this really the intended behaviour or might this be a bug?