# #276 remainder — SpawnPlacementSettler settle-cell discard: scoping (2026-08-06)

Analysis complete; **no production change committed.** A candidate fix was
written, built clean, and passed the three existing settler tests — then
**deliberately reverted**, because the only test that would *discriminate* it
needs an EnvCell fixture that was not safe to assemble in the session that
found this. See §4.

Written at HEAD `1d2d4bb8`.

---

## 1. The headline finding — the issue's framing is half right, and the half it misses is the whole fix

`docs/ISSUES.md:1640` says the settler "commits `settle.Position` but never
reads `settle.CellId`", so a settle crossing a cell boundary "leaves the body's
cell at the placement cell". The symptom is real. The **outdoor** half of the
explanation is not.

`PhysicsBody.Position`'s ordinary setter already carries the world displacement
into the landblock-relative frame **and** calls `LandDefs.AdjustToOutside`,
which recomputes the outdoor cell index from the local origin across 24 m cell
crossings and wraps/bumps the landblock across 192 m boundaries
(`src/AcDream.Core/Physics/PhysicsBody.cs:279-282`). So for an outdoor→outdoor
settle, `body.Position = settle.Position` **already lands the correct cell**.
Discarding `settle.CellId` costs nothing there.

**The real gap is EnvCells.** An EnvCell id is not derivable from a world
position — there is nothing for `AdjustToOutside` to recompute, and its guard
(`(cell & 0xFFFF) is >= 1 and <= 0x40`, `PhysicsBody.cs:266`) deliberately
excludes EnvCell ids (≥ 0x100) from that path entirely. The resolver's
`settle.CellId` is the *only* carrier of an EnvCell identity, and it is exactly
what the settler drops. So the live defect is the issue's parenthetical —
"outdoor/EnvCell seam, stacked EnvCells" — and not its main clause.

This matters for the fix's risk profile: the change is a **no-op on the outdoor
path that dominates production**, and corrective only at the indoor seam.

## 2. The fix (validated, then reverted)

Replace `body.Position = settle.Position;`
(`src/AcDream.Core/Physics/SpawnPlacementSettler.cs:61`) with the identical
guarded shape the per-tick resolve writeback uses at
`src/AcDream.Runtime/Physics/RuntimeOrdinaryPhysicsUpdater.cs:162-167`:

```csharp
uint resolvedCellId = settle.CellId != 0 ? settle.CellId : cellId;
body.CommitTransitionPosition(resolvedCellId, settle.Position);
```

This is the issue's own prescribed shape ("commit the settle's resolved cell
through the same body/cell channel the per-tick resolve writeback uses").

**Why the ordering is safe** — worth recording, because reading
`CommitTransitionPosition` (`PhysicsBody.cs:257-277`) in isolation suggests it
pairs the *new* cell with a *stale* local origin. It does not: line 259's
`Position = worldPosition` runs the ordinary setter first, which updates
`CellPosition.Frame.Origin`; line 265 then reads the **already-updated** origin.
Retail anchor: `CPhysicsObj::SetPositionInternal(CTransition const*)`
@0x00515330 commits both `sphere_path.curr_pos.objcell_id` and its frame,
including EnvCells.

**Verified:** `dotnet build src/AcDream.Core -c Release` → 0 errors, 0 warnings;
`SpawnPlacementSettlerTests` 3/3 pass.

## 3. Reach — one caller confirmed, one unverified

| caller | passes as `cellId` | body carries a cell? |
|---|---|---|
| `RuntimeLocalPlayerPhysicsPublicationState.cs:812` (C3c local first-entry) | `activation.Body.CellPosition.ObjCellId` | **Yes**, by construction |
| `LiveEntityNetworkUpdateController.cs:297` (remote spawn seed, #270) | a `cellId` parameter | **UNVERIFIED** |

`CommitTransitionPosition` early-returns when `CellPosition.ObjCellId == 0`
(`PhysicsBody.cs:261`), so on any body without a cell the fix is an inert no-op
— safe, but it would mean #276 stays open for remotes even after the change.
**Establish the remote body's `CellPosition` provenance before claiming the fix
closes both halves.** The register row AD-61 covers this shared settler.

## 4. Why no test was committed, and the exact test to write

The three existing tests
(`tests/AcDream.Core.Tests/Physics/SpawnPlacementSettlerTests.cs`) construct
bodies with **no `CellPosition`**, so every one of them passes identically with
or without the fix — they cannot discriminate it. Shipping the change against
them would repeat the defect this campaign already paid for once (a
conservation test that passed with its own change reverted, C5b finding D3).

Per §1 the discriminating case must be **indoor**, because an outdoor fixture
also passes both ways via `AdjustToOutside`. Required shape:

1. `AddLandblock(0xA9B40000, flat terrain, [envCellSurface], portalPlanes, …)`
   with an EnvCell surface id ≥ `0x100` — build the floor with
   `CellSurfaceTests.MakeFlatFloor`'s 4-vertex quad pattern
   (`tests/AcDream.Core.Tests/Physics/CellSurfaceTests.cs:20-38`).
2. Body seeded with `CellPosition.ObjCellId` = an **outdoor** placement cell
   (e.g. `0xA9B40001`), positioned just above the EnvCell floor.
3. Call `TrySettle` with that outdoor cell as the wire `cellId`.
4. **Assert** `body.CellPosition.ObjCellId == 0xA9B40100` (the EnvCell).
5. **Sabotage that must redden it:** restore `body.Position = settle.Position`
   — the cell stays outdoor, because `AdjustToOutside` cannot invent an EnvCell.

The open risk, and the reason this was not attempted at the end of a long
session: step 1 must produce a fixture where the resolver genuinely reports the
EnvCell in `settle.CellId`. That may require portal planes for cell membership
(`CellTransit.FindCellList`). A fixture that silently resolves to the outdoor
cell would make the test pass for the wrong reason in the *other* direction —
green, and proving nothing. Verify `settle.CellId` is the EnvCell id before
trusting the assertion.

## 5. Recommended next step

Write the §4 test first, watch it fail against HEAD, then apply §2 and watch it
pass, then sabotage. One commit, ~40 lines of test, ~8 of production. Resolve §3
in the same pass or state plainly that the remote half remains open.