Hardware flow control will be enabled appropriately when rts or cts are defined. The rts-threshold is set 1, resulting in a rxbuf + 1, before rts is raised. Then still FIFO_size - 1 bytes can be received without data loss. The FIFO_size is 128.
Note: Setting rts_threshold to 0 has a strange side effect, that rts pulses with every received byte.