code: def extend_left_to(self, window, max_size):
          
          rofs = self.ofs - window.ofs_end()
          nsize = rofs + self.size
          rofs -= nsize - min(nsize, max_size)
          self.ofs = self.ofs - rofs
          self.size += rofs
docs: Adjust the offset to start where the given window on our left ends if possible,
          but don't make yourself larger than max_size.
          The resize will assure that the new window still contains the old window area