CONTROL: XOFF (0x13) received by master
CONTROL: XON (0x11) after slave read-drain: SEEN (expected)
TEST: XOFF (0x13) received by master; TS_TBLOCK set
TEST: TIOCFLUSH(FREAD|FWRITE) done, master drained
TEST: XON (0x11) after TCIOFLUSH: NOT SEEN => XON discarded by ttyflush (BUG)
TEST: post-flush input still flows (read 64 bytes) while resume char is lost
RESULT: BUG REPRODUCED (XON discarded by ttyflush)