“The overlap broke — so I shut the door”
The situation
Blue is the outside overlapped boat as the two reach the leeward-mark zone, so she owes Green mark-room. Coming in, Blue drops back, the overlap breaks, and Blue closes the door at the mark, sure she now owes nothing. The boats touch and Blue protests Green for forcing in.
By the mark the overlap is gone and Green is effectively astern, so Green has no inside rights and barged in — Blue's protest looks correct.
The question
The overlap really did break — doesn't that release Blue?
The ruling
No — Blue is penalized. Mark-room is locked in at the moment the first boat reaches the zone, and if a boat was overlapped outside then, she must keep giving mark-room even if the overlap later breaks (rule 18.2(a)). Blue owed Green room for the whole rounding; closing the door was the foul. Green, taking the room she was still owed, is fine. The broken overlap is a trap — the clock that matters stopped at the zone.
Overlapped outside at the zone? You owe mark-room the whole rounding — a later broken overlap doesn't free you (rule 18.2(a)).
Opens the situation on the boat-length grid — scrub it and see exactly how the boats meet. Free, no account needed.
Rules cited
See the underlying rule: Rule 18.2 — Mark-Room — Overlap Breaks.