docs: def showPopup( self ):
          """
          Displays a custom popup widget for this system if a checkable state \
          is setup.
          """
          if not self.isCheckable():
              return super(XComboBox, self).showPopup()
          
          if not self.isVisible():
              return
          
          # update the checkable widget popup
          point = self.mapToGlobal(QPoint(0, self.height() - 1))
          popup = self.checkablePopup()
          popup.setModel(self.model())
          popup.move(point)
          popup.setFixedWidth(self.width())
          
          height = (self.count() * 19) + 2
          if height > 400:
              height = 400
              popup.setVerticalScrollBarPolicy(Qt.ScrollBarAlwaysOn)
          else:
              popup.setVerticalScrollBarPolicy(Qt.ScrollBarAlwaysOff)
          
          popup.setFixedHeight(height)
          popup.show()
          popup.raise_()