autoscroll on drag

Hello!
Help me please what's going on:
i want to have auto-scroll in my QListWidget during drag-n-drop operation, so when the dragged item is at the bottom or at the top of viewport, the listwidget should scroll. Everything is ok by default - setAutoScroll(True), setAcceptDrops(True), etc.

When i add my code: (anything unusual, just for me)

Code:

---

def startDrag(self):
        items = self.selectedItems()
        if bool(items):
            icon = items[0].icon()
            data = QByteArray()
            stream = QDataStream(data, QIODevice.WriteOnly)
            stream << QString(', '.join(map(lambda x: unicode(x.text()), items)))
            mimeData = QMimeData()
            mimeData.setData("application/x-text", data)
            drag = QDrag(self)
            drag.setMimeData(mimeData)
            pixmap = icon.pixmap(24,24)
            drag.setHotSpot(QPoint(12,12))
            drag.setPixmap(pixmap)
            drag.exec_(Qt.MoveAction)
 
def mouseMoveEvent(self, ev):        
        self.startDrag()
        QWidget.mouseMoveEvent(self, ev)

---

the autoscroll disappears. Why? Should i write my own auto-scroll, catch the point at the top and so on? I want to code less)
So, what's the reason?