code: def _screen_size_cb(self, w, tf):
          
          self._screen_size = tf
          self.w.width.set_enabled(not tf)
          self.w.height.set_enabled(not tf)
          self.w.lock_aspect.set_enabled(not tf)
          if self._screen_size:
              wd, ht = self.fitsimage.get_window_size()
              self._configure_cb(self.fitsimage, wd, ht)
docs: This function is called when the user clicks the 'Screen size'
          checkbox.  `tf` is True if checked, False otherwise.