OS X: transient scrollbars do not fade out

#include <QtWidgets>

static QScrollArea* createScrollArea(QWidget* parent = 0)
{
    QScrollArea* sa = new QScrollArea(parent);

    QWidget* content = new QWidget(sa);
    content->resize(2048, 2048);

    QLinearGradient gradient(0, 0, 2048, 2048);
    gradient.setColorAt(0, "lightblue");
    gradient.setColorAt(1, "pink");

    QPalette pal;
    pal.setBrush(QPalette::Window, gradient);
    content->setPalette(pal);
    content->setAutoFillBackground(true);

    sa->setWidget(content);
    return sa;
}

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    QTabWidget widget;
    widget.resize(640, 480);
    widget.addTab(createScrollArea(&widget), "A");
    widget.addTab(createScrollArea(&widget), "B");
    widget.show();
    return app.exec();
}