Pinned (flap-sidechk.log): the indoor doorway flap is a 0171<->0173 flood cycle. Back portals show camInterior=False (our side test already agrees with retail) but were traversed when eyeIn=True because the side-cull had an bypass (added 2026-06-05 for the void). Within 1.75m of a doorway that bypass kept the BACK portal alive -> mutual re-contribution -> re-enqueue churn (maxPop=16) -> eye-sensitive flood depth -> grey flap + dropped floor.
Fix (Option B1): drop the bypass from the side-cull in Build + BuildFromExterior so back portals cull by the side test alone, exactly like retail PView::InitCell (:432962, no eye-in-opening bypass). The forward-portal clip-empty void rescue is a SEPARATE branch and is untouched (Build_EyeStandingInInteriorPortal_FloodsNeighbour stays green). New RED->GREEN test Build_BackFacingPortal_EyeStandingInOpening_StillCulled; full App suite 218 green.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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