why item corrdinate is incorrect inside QgraphicsScene?

[hashb]

Hi All,

view_ = new QGraphicsView(this);
    qDebug()<<view_->horizontalScrollBar()->height(); //30
 
    view_->resize(
        200 + view_->horizontalScrollBar()->height(),
        200+view_->horizontalScrollBar()->height()
    );
    scene_ = new QGraphicsScene(0,0,200,200,this);
    view_->setScene(scene_);
 
    QGraphicsRectItem *rect = new QGraphicsRectItem(QRect(0,0,20,20));
    rect->setBrush(Qt::red);
    rect->setPos(0,0);   
    scene_->addItem(rect);
 
 
    QPushButton *bt=new QPushButton("ok",this);
    bt->move(15,15);

view_ = new QGraphicsView(this);
    qDebug()<<view_->horizontalScrollBar()->height(); //30

    view_->resize(
        200 + view_->horizontalScrollBar()->height(),
        200+view_->horizontalScrollBar()->height()
    );
    scene_ = new QGraphicsScene(0,0,200,200,this);
    view_->setScene(scene_);

    QGraphicsRectItem *rect = new QGraphicsRectItem(QRect(0,0,20,20));
    rect->setBrush(Qt::red);
    rect->setPos(0,0);   
    scene_->addItem(rect);


    QPushButton *bt=new QPushButton("ok",this);
    bt->move(15,15);

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since default alignment of scene is center,
and the view's size is (230,230)
the scene's size is (200,200)
so I guess ,the rect should at QPoint(15,15),
I place a button at QPoint(15,15) to check whether the button can cover the rect totally,
but from the attached pic ,we can see there is a gap between them ,
I was wondering why, any ideas about it?

Thank advance for your help.
Best regards,
hb