WebAssembly: QScrolllArea always updates with refreshing the entire viewport

#include <QApplication>
#include <QDebug>
#include <QMainWindow>
#include <QPaintEvent>
#include <QPainter>
#include <QScrollArea>

class MyWidget : public QWidget
{
public:
    MyWidget() {}

    void paintEvent(QPaintEvent *event)
    {
        QPainter painter(this);
        painter.fillRect(event->rect(), Qt::white);
        qDebug() << event->rect();
    }
};

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

    MyWidget *widget = new MyWidget();
    widget->resize(2000, 3000);
    QScrollArea *scrollArea = new QScrollArea();
    scrollArea->setWidget(widget);

    QMainWindow w;
    w.setCentralWidget(scrollArea);
    w.show();
    return a.exec();
}