QSlider handle in plastique style does not look good with non constant brackground

With QPlastiqueStyle, the tip of the slider handle of a QSlider does not look good because it is not transparent when using a non-constant background. The tip should have been drawn transparent, but instead it uses part of the window background.

Reproducible with the following code:
#include <QtGui>
#include <QDebug>

int main(int argc, char *argv[])
{
QApplication::setStyle(new QPlastiqueStyle ());
QApplication app(argc, argv);
QSlider slider;

QPalette p;
p.setBrush( QPalette::Window, QBrush( QPixmap("/1.jpg") ));
slider.setAutoFillBackground(true);
slider.setPalette(p);
slider.show();
return app.exec();
}