code: def mode_key_up(self, viewer, keyname):
          
          # Is this a mode key?
          if keyname not in self.mode_map:
              # <== no
              return False
  
          bnch = self.mode_map[keyname]
          if self._kbdmode == bnch.name:
              # <-- the current mode key is being released
              if bnch.type == 'held':
                  if self._button == 0:
                      # if no button is being held, then reset mode
                      self.reset_mode(viewer)
                  else:
                      self._delayed_reset = True
              return True
  
          return False
docs: This method is called when a key is pressed in a mode and was
          not handled by some other handler with precedence, such as a
          subcanvas.