# Handoff/Findings — the "physics rest µm-jitter" flap diagnosis is REFUTED; the flap is a RENDER membership instability at the grazing doorway portal — 2026-06-08 (PM)

> **Supersedes** `docs/research/2026-06-08-flap-rootcause-physics-rest-handoff.md` (the AM handoff). That
> handoff asked to be treated as a suspect statement and verified — this session verified it with primary
> evidence and it does **not** hold. The flap is **not physics** and **not camera drift**. It is the
> portal-flood **membership flickering non-monotonically at the grazing doorway portal as the camera eye
> sweeps** (i.e. while you turn the camera at the doorway). This is the SPEC's §2.2 diagnosis
> (`2026-06-08-portal-flood-membership-stability-design.md`), NOT its refuted §4 enqueue-once fix.

---

## 1. What was claimed (AM handoff) vs what the evidence shows

**AM claim:** the flap's varying input is a **physics resting-position µm jitter** — `_body.Position`
blips ~1 ULP between ticks at rest → `RenderPosition` (Lerp of physics) jitters → eye jitters → flood
flips. Fix = physics rest-stability (broaden `kill_velocity`, hold contact plane).

**Refuted by three independent pieces of primary evidence:**

### A. The physics body is bit-stable at standstill — it does NOT blip
`door-recheck-capture.jsonl` (515 MB, 238,342 `ResolveWithTransition` records, captured standing at the
doorway, cells `0xA9B40170/0171/0174/0175/0031`):
- **216,300 true-standstill records** (zero velocity, `currentPos==targetPos`).
- **0** resolve re-snaps (`result.position != input` never happens at standstill).
- **0** cross-tick `currentPos` drift (the body position is carried forward byte-identically).
- The `grounded-but-cp=none` contradictory state DOES occur (3.5% of frames) but produces **no** position
  blip.

Confirmed independently with **4 new deterministic tests** (all GREEN — they PROVE rest is bit-stable):
- `PlayerMovementControllerTests.Update_AtRestNoInput_RenderPositionBitStableAcrossManyFrames` (flat terrain)
- `PlayerMovementControllerTests.Update_WalkThenStop_SettlesToBitStableRest` (flat terrain, post-motion)
- `CellarUpTrajectoryReplayTests.IndoorCellarFloor_AtRestZeroOffset_BodyPositionBitStable` (indoor cell, resolver loop)
- `CellarUpTrajectoryReplayTests.IndoorCell_FullController_AtRestNoInput_RenderPositionBitStable` (indoor cell, full controller loop)

### B. On the actual flap frames, the PLAYER position is byte-identical
`pvinput.log` window 7748–7758 (a clean `flood 6↔2` flap), player `RenderPosition` for **11 consecutive
frames**: `(155.632858, 13.527222, 94.000000)` — **byte-identical**. The physics output the camera reads
does not move at all during the flap. (The 1-ULP *player* blip the AM handoff cited is at the **outdoor**
`flood=1` records — a red herring, not the indoor flap.)

### C. The EYE moves while the player is still — and the camera DOES settle when idle
Same window: the eye orbits smoothly ~1 mm/frame (X ↓, Y ↑, Z constant) — a slow **camera rotation**
around the stationary player. And:
- Of **888** flood flips in the capture, only **1** had a byte-identical eye (and that one is the
  outdoor→indoor root switch). Every other flip had a moved eye → **the flood is deterministic in the
  eye; it changes only when the eye moves** (matches `Build_IsDeterministic_*`).
- Longest **indoor byte-identical-eye runs: 203 / 181 / 178 frames (~3.4 s)** — within each, the flood is
  a **single constant value** (no flicker). **61%** of indoor frames have a byte-stable eye.
- ⇒ The camera **settles** at rest (no boom drift, no spring oscillation). When the eye is still, the
  flood is stable. The flap fires **only while the eye is moving**.

## 2. The actual mechanism

When the camera eye **sweeps** through the grazing doorway portal (you turn the camera at the threshold),
the deep cell cluster `{0172,0173,0174,0175}` flickers in/out — flood `6,6,6,2,2,6,6,6,2,6,2` — i.e.
**non-monotonic membership across a monotonic eye sweep**. A correct visibility flood would transition
the deep cluster in/out **once** as the grazing portal closes; instead its clip flips empty↔non-empty as
the eye crosses and re-crosses the knife-edge. This is the SPEC's §2.2 diagnosis (the grazing portal's
clip / re-clip drift makes `clippedRegion.Count` flip `0↔N`, dropping the deep cluster on empty-clip
frames).

It is **NOT** physics (A, B). It is **NOT** camera drift/oscillation (C: eye byte-stable ~3.4 s when
idle). It is a **render-side portal-flood membership instability at grazing angles**, surfaced by camera
rotation.

## 3. Status of prior fixes / diagnoses

- **AM physics-rest fix** — would not have fixed the flap (physics rest is already bit-stable). Do not pursue.
- **SPEC §2.2 diagnosis (grazing-portal membership instability)** — CONFIRMED by this evidence.
- **SPEC §4 enqueue-once fix** — already refuted in the AM handoff (retail propagates late slices via
  `AddToCell`, decomp :433494; broke `Build_ViewGrowthAfterDoneCell_PropagatesNewSlicesToExit`). So the
  correct fix is a render-side membership stabilization that is monotonic under a sweeping eye **without**
  breaking late-slice propagation — design TBD (brainstorm).

## 4. Apparatus (this session)
- 4 GREEN rest-stability tests (above) — keep as regression guards + evidence that physics rest is bit-stable.
- Analysis scripts (ad-hoc, `/tmp`): door-recheck standstill survey; pvinput flood-flip vs eye/player
  delta buckets; indoor byte-stable-eye-run scan. Re-derivable from `pvinput.log` + `door-recheck-capture.jsonl`.
- Existing probes `[pv-input]` (`ACDREAM_PROBE_PVINPUT`) and `[render-sig]` remain the live gate.

## 5. Next step (proposed)
Brainstorm + design a render-side fix that makes the deep-cluster membership **monotonic/stable as the eye
sweeps the grazing portal** (candidates: more robust grazing-portal clip, a retail-faithful single-process
traversal that doesn't re-clip-drift, or matching retail's exact `GetClip`/`polyClipFinish` epsilon). Then
TDD a builder test that sweeps the eye across the grazing angle and asserts monotonic membership, and
visual-gate by turning the camera at the cottage doorway.

## 6. LIVE-CONFIRMED (2026-06-08 PM, targeted doorway capture, user-driven)

A fresh instrumented capture (`launch-flap-capture.ps1`; `[pv-input]` enhanced with `rawPlayer`=raw
physics body pos + `yaw`; cells `0170-0175` dumped to `tests/.../Fixtures/flap-doorway/`; 84K frames)
confirms the diagnosis across every state and decomposes the flap into THREE render sub-issues. **In
every case the player render-pos AND raw physics-pos are byte-identical (0 µm) — physics is conclusively
exonerated; the flap is 100% camera-eye-driven.**

| State (user-driven) | player moves | eye moves | flood |
|---|---|---|---|
| Idle, hands fully off | no (0 µm) | **no (0 µm)** | **stable** (no flap) |
| Turn / walk | no (0 µm) | yes (mm, yaw) | oscillates |
| Camera smoothing-glide after a turn (yaw byte-constant, eye glides monotonically, decelerating) | no | yes (mm) | **oscillates 8↔3** ← this is the "flickers while idle" the user perceived |

Key burst (row 11167): **yaw byte-constant**, eye X glides monotonically 155.109→155.435 (18→5 mm/frame,
decelerating), flood `8→3→8…3→8`. Monotonic eye ⇒ non-monotonic membership ⇒ **render** instability (not
camera hunting).

**Three sub-issues (all eye-driven, physics out):**
- **A — Membership oscillation:** flood non-monotonic as the eye sweeps within a *stable* root.
  outside-looking-in `8↔3`; outdoor-root `17↔33` (21 flips/2500 frames); indoor-root `2↔6`.
- **B — Root toggle (the big one):** at the threshold, `outRoot` flips outdoor↔indoor as the eye crosses
  the door plane → wholesale visible-set swap **≈18-33 cells ↔ 2 cells** (4 toggles in 2500 frames). This
  is the "two-branch" outdoor-node-vs-indoor-cell root switch the unification was meant to remove — still
  present.
- **C — Indoor-root under-inclusion:** eye just inside ⇒ `outRoot=n` flood **= 2, stable** for 2438
  frames → outdoors + other rooms missing (the indoor flood does not reach back out the exit portal / to
  adjacent cells). C is B's partner: the swap *to* indoor loses the scene → "textures missing."

**Fix scope:** core render pipeline (root resolution + flood + grazing-clip), NOT physics, NOT camera.
Spec §2.2 (membership instability) is right for A; B+C are the threshold root-resolution/flood issues.
Spec §4 enqueue-once stays refuted. Design needs brainstorming (saga has reverted speculative render
fixes — see `feedback_render_one_gate`, `feedback_verify_render_seal_before_layering`).

Apparatus added: `launch-flap-capture.ps1`, `analyze_flap_live.py`, `find_burst.py`, fixtures
`tests/AcDream.Core.Tests/Fixtures/flap-doorway/0xA9B4017{0..5}.json`, `flap-doorway-resolve.jsonl`.

**Memory to correct:** `project_indoor_flap_rootcause` (root is render: A membership instability + B
root-toggle + C indoor under-inclusion, all under a moving camera eye — NOT physics rest, NOT camera
drift; the "two-branch split" B is still live).