code: def on_scroll(self):
          
          try:
              sw = self.view['sw_scroller']
          except KeyError:
              return False # destroyed!        
          vadj = sw.get_vadjustment()
          if vadj is None: return False
          val = vadj.get_value()
          
          # is scrolling over?
          if val >= vadj.upper - vadj.page_size:
              self.view.show_vscrollbar()
              return False
          
          vadj.set_value(val+0.5)
          return True
docs: Called to scroll text