I found the reason for the behaviour: In UIGraphicsScene::mouseReleaseEvent I forgot to pass the mouseEvent on to QGraphicsScene::mouseReleaseEvent:

Qt Code:
  1. void UIGraphicsScene::mouseReleaseEvent(QGraphicsSceneMouseEvent *mouseEvent)
  2. {
  3. emit insertFinished();
  4. QGraphicsScene::mouseReleaseEvent(mouseEvent);
  5. }
To copy to clipboard, switch view to plain text mode