code: def display_current(self):
          
          if self.idx_current in self.scene.items():
              self.scene.removeItem(self.idx_current)
  
          item = QGraphicsRectItem(0,
                                   CURR['pos0'],
                                   self.parent.value('window_length'),
                                   CURR['pos1'])
          # it's necessary to create rect first, and then move it
          item.setPos(self.parent.value('window_start'), 0)
          item.setPen(QPen(Qt.lightGray))
          item.setBrush(QBrush(Qt.lightGray))
          item.setZValue(-10)
          self.scene.addItem(item)
          self.idx_current = item
docs: Create a rectangle showing the current window.