QScrollArea: The viewport bleeds through another widget when the scroll bar is reset.

#include <QApplication>
#include <QScrollArea>
#include <QPainter>
#include <QPaintEvent>
#include <QScrollBar>

class CustomWidget : public QWidget
{
public:
    CustomWidget(const QColor &b, QWidget *parent = 0)
        : QWidget(parent), background(b)
    {
    }
    
    void paintEvent(QPaintEvent *e)
    {
        QPainter p(this);
        p.fillRect(e->rect(), background);
    }

    QColor background;
};

class MainWidget : public QWidget
{
public:
    MainWidget(QWidget *parent = 0)
    : QWidget(parent)
    {
        area = new QScrollArea(this);
        area->setWidget(new CustomWidget(Qt::gray));

        widget = new CustomWidget(Qt::red, this);

        timerId = startTimer(100);
    }

    void resizeEvent(QResizeEvent *e)
    {
        QWidget::resizeEvent(e);

        area->resize(800, 100);
        area->widget()->resize(5000, 600);
        area->move((width() - area->width()) / 2, (height() - area->height()) / 2);

        widget->resize(500, 200);
        widget->move((width() - widget->width()) / 2, (height() - widget->height()) / 2);
    }

    void timerEvent(QTimerEvent *e)
    {
        if (e->timerId() == timerId) {
            if (area->horizontalScrollBar()->value() == area->horizontalScrollBar()->maximum())
                area->horizontalScrollBar()->setValue(0);
            else
                area->horizontalScrollBar()->setValue(area->horizontalScrollBar()->value() + 200);
        } else {
            QWidget::timerEvent(e);
        }
    }

private:
    QScrollArea *area;
    CustomWidget *widget;
    int timerId;
};

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    MainWidget w;
    w.showMaximized();
    return app.exec();
}