Unwanted margin inside QGraphicsView with Scrollbars

[lpfxyww]

Hi,
I'm drawing something with the Qgraphicsview framework. The view hold a scene with a single Item. The scene's height is equivalent to the item's. But when I try to resize the view to its minimum height, there's a gap(the green part) between the content and the view's boarder, shown in the attached photo.
Here is a simpler version of my code（I replaced the item with a pixmap）:

#include <QtGui/QApplication>
#include <QMainWindow>
#include <QGraphicsScene>
#include <QGraphicsView>
 
int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    QMainWindow window;
 
    QPixmap pixmap(200, 200);
    pixmap.fill(QColor(255, 0, 0));
 
    QGraphicsScene scene(&window);
    scene.addPixmap(pixmap);
 
    QGraphicsView view(&window);
    view.setBackgroundBrush(QColor(0, 255, 0));
    view.setScene(&scene);
 
    window.setCentralWidget(&view);
    window.show();
    window.resize(199, 199);
 
    return app.exec();
}

#include <QtGui/QApplication>
#include <QMainWindow>
#include <QGraphicsScene>
#include <QGraphicsView>

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    QMainWindow window;

    QPixmap pixmap(200, 200);
    pixmap.fill(QColor(255, 0, 0));

    QGraphicsScene scene(&window);
    scene.addPixmap(pixmap);

    QGraphicsView view(&window);
    view.setBackgroundBrush(QColor(0, 255, 0));
    view.setScene(&scene);

    window.setCentralWidget(&view);
    window.show();
    window.resize(199, 199);

    return app.exec();
}

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Anyone can help me? Thanks