windows specific: point is not painted if set to Qt::FlatCap or Qt::SquareCap

The following code should paint some black points into the pixmap, but it doesn't work on qt4.6 windows version.
QPainter painter(&m_image);
painter.setRenderHints(QPainter::Antialiasing|QPainter::SmoothPixmapTransform);
painter.setPen(QPen(QColor(0,0,0),10));
painter.setBrush(Qt::NoBrush);
painter.drawPoint(e->pos());

If you set the pen cap to Qt::RoundCap, it draws.

This only happens in windows version. and there is no such problem against 4.5.x.
test example attached.