Patched kernel (DragonFly 6.5-DEVELOPMENT #1: Mon Oct 12 20:09:42 UTC 2026),
unprivileged uid=1001 (maxx), corrected PoC:

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 pre-drained)
TEST: XON (0x11) after TCIOFLUSH: SEEN (bug NOT reproduced)
TEST: post-flush input still flows (read 64 bytes) while resume char is lost
RESULT: BUG NOT REPRODUCED
exit=1
=> with fix.diff the resume character survives the flush: defect GONE.