main.cpp

#include <QtGui/QPainter>
#include <QtWidgets/QApplication>
#include <QtWidgets/QScrollArea>

class Widget : public QWidget
{
    Q_OBJECT

public:
    explicit Widget(QWidget *parent = 0)
        : QWidget(parent)
    {
        resize(200, 200);
    }

protected:
    void paintEvent(QPaintEvent *)
    {
        QPainter p(this);
        p.fillRect(-100, 100, 400, 400, Qt::blue);
        p.fillRect(rect(), Qt::red);
    }
};

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

    QScrollArea scrollArea;
    scrollArea.setBackgroundRole(QPalette::Dark);
    scrollArea.setAlignment(Qt::AlignCenter);
    scrollArea.setWidgetResizable(false);
    scrollArea.setWidget(new Widget);
    scrollArea.resize(400, 400);
    scrollArea.show();

    return app.exec();
}

#include "main.moc"