Overlap obtained too late
The situation
Blue is clear astern as Green reaches the zone, then ranges up and gets an inside overlap once they're already inside the zone, and calls for room.
The question
Does the overlap she made inside the zone entitle Blue to mark-room?
The ruling
No. Mark-room is locked in the moment the first boat reaches the zone. Blue was clear astern then, so she gets no mark-room even though she later gets an inside overlap inside the zone (rule 18.2(a)(2) — and the obligation continues even when a new overlap begins). Green keeps her room; Blue must keep clear and round behind. To earn the inside, you must have the overlap clearly established before the zone. The animation shows Blue's late overlap getting shut out.
An inside overlap made after the zone gives you no mark-room — get it established before the zone or round behind (rule 18.2(a)(2)).
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 Too Late.