test #137: the seam shake reproduced offline — third mechanism characterized

The corridor gate FAILED with a changed symptom: shaking at cell seams
(+ purple floor flashing there) instead of the dead stop. Deep-probe
session (step-walk/push-back/indoor-bsp + full resolve capture) traced
the complete chain; docs/ISSUES.md #137 carries it. Short form:

- Corridor floors are double-faced portal slabs over under-rooms; the
  resting foot sphere lives within half a millimeter of three hit/straddle
  thresholds there.
- Crossing a boundary, the foot penetrates the neighbor ramp slab by
  ~0.4mm, steps up onto it successfully (+0.6mm lift, stepped=True) —
  and the lifted check position is then LOST: the following pass runs at
  the unlifted height (the P2 stale-snapshot class; retail step_up
  0x0050b6cc restores only on FAILURE).
- The unlifted re-test grazes the under-room's ceiling (the slab
  underside) within the near-miss window, dispatches a neg-poly step-up
  with a DOWNWARD normal, whose nested step-down finds no walkable at
  exact tangency -> StepUpSlide -> slide_sphere opposing branch ->
  reversed-movement collision -> Collided -> revert. Every frame = shake.

Apparatus committed:
- Issue137CorridorSeamReplayTests: 3 deterministic offline repros
  (snapshot-exact west-boundary from the capture, east deep-straddle,
  the clean-run pin), currently Skip='#137 seam shake' pending the fix.
  Key: THREE portal-ring hydration (the under-room 0x8A020166 is ring-3;
  with fewer rings the flood can't add it and the bug vanishes) + live
  Setup step heights (0.6/1.5) + probe-buffer capture for line-diffing
  offline vs live traces.
- Issue137CorridorSeamInspectionTests: portal-poly world spans (exposed
  the visual-vs-physics polygon-id conflation and the floor-portal
  topology), physics-BSP leaf membership walk, hit-normal candidate
  sweep (|align| both windings), downward-poly sweep.

NEXT: read TransitionalInsert's attempt loop against retail 0x0050b6f0,
find the restore that clobbers the successful step-up position, fix,
un-skip. The purple seam flashing is expected to be the render exposing
the same per-frame oscillation - re-check after the physics fix.

Suites: Core 2553 / App 713 / UI 425 / Net 385, 0 failures (5 skips =
2 pre-existing + the 3 parked repros).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/ISSUES.md

@@ -1183,8 +1183,46 @@ walls** in particular. (Symptoms not fully characterized yet: likely walking thr
 openings that should block / blocking at openings that should pass, and door collision not
 matching the door's open/closed state.)
 
-**CORRIDOR PHANTOM FIXED 2026-07-06 (both mechanisms resolved; visual gate
+**GATE 2026-07-06 FAILED — THIRD MECHANISM CHARACTERIZED (the seam shake),
-pending — see `docs/research/2026-07-06-137-sliding-normal-lifecycle-audit.md`
+deterministic offline repro secured:** with mechanisms 1+2 fixed the dead
+stop became a SHAKE at cell seams (+ purple floor flashing there — almost
+certainly the render exposing the same per-frame position/OnWalkable
+oscillation; re-check after the physics fix). Full chain, every link
+probe-traced (`launch-137-seam-probes.log`, capture
+`resolve-137-seam-capture.jsonl` tick 4101 ×46):
+1. Corridor cells sit above under-rooms; the shared floor slab is
+   double-faced (up-face + underside as separate physics polys) and IS a
+   portal plane (e.g. 0x8A020165's ramp over 0x8A020166). The resting foot
+   sphere is permanently within ±0.5 mm of THREE thresholds there (poly-hit
+   r−ε, walkable r−ε, portal-straddle r+ε).
+2. Walking across the boundary at the flat-floor height penetrates the
+   ramp slab by ~0.4 mm → foot full-hit on the up-face → StepSphereUp →
+   step-down accepts the ramp (+0.6 mm lift, CheckPos −5.999,
+   `[stepsphereup] stepped=True`).
+3. **THE BUG: the lifted position is then LOST** — the next pass runs at
+   the UNLIFTED height (GlobalSphere center −5.520 vs the lifted −5.519;
+   the P2 stale-snapshot class, single-slot Save/RestoreCheckPos clobber
+   suspected — retail `CTransition::step_up` 0x0050b6cc restores ONLY on
+   failure) → the re-test at 0.4 mm inside the slab grazes the NEIGHBOR
+   under-room's CEILING (the slab underside, n≈(−0.03,0,−1)) within the
+   near-miss window → recorded (retail records it too — pos_hits_sphere
+   registers geometric hits pre-cull) → neg-poly step-up dispatch with the
+   DOWNWARD normal → the nested step-down finds no walkable at exact
+   tangency → StepUpSlide → slide_sphere(down normal vs up contact plane)
+   → the opposing branch → reversed-movement collision normal → Collided →
+   validate revert (Contact/OnWalkable stripped) → next step's AdjustOffset
+   zeroes → out==in every frame = the shake. Retail never enters at step 3:
+   its kept step-up lift leaves the sphere ON the surface, no graze.
+4. Offline repro: `Issue137CorridorSeamReplayTests` (3 tests, currently
+   `Skip="#137 seam shake"`) reproduce the block deterministically — the
+   key was hydrating THREE portal rings (the under-room 0x8A020166 is
+   ring-3; with fewer rings the flood can't add it and everything passes).
+   NEXT: read our `TransitionalInsert` attempt loop against retail
+   0x0050b6f0 to find the restore that clobbers the successful step-up's
+   position; fix; un-skip the three tests.
+
+**CORRIDOR PHANTOM mechanisms 1+2 FIXED 2026-07-06 (see
+`docs/research/2026-07-06-137-sliding-normal-lifecycle-audit.md`
 for the full audit):** mechanism 2 = BSPQuery Contact-branch stub slide
 responses leaked sliding normals retail's BSP layer never writes (fixed:
 real `slide_sphere` routing + success-gated body writeback). Mechanism 1 as

tests/AcDream.Core.Tests/Physics/Issue137CorridorSeamInspectionTests.cs

@@ -28,6 +28,7 @@ public class Issue137CorridorSeamInspectionTests
     [Theory]
     [InlineData(0x8A02016Eu)]
     [InlineData(0x8A02017Au)]
+    [InlineData(0x8A02011Eu)]   // the under-floor room the corridor's floor-portals lead to
     public void CorridorCell_PhysicsPolysAndPortals_DatInspection(uint envCellId)
     {
         var datDir = Env.GetEnvironmentVariable("ACDREAM_DAT_DIR")
@@ -254,7 +255,10 @@ public class Issue137CorridorSeamInspectionTests
                     System.Numerics.Vector3.Cross(w1 - w0, w2 - w0));
 
                 float align = System.Numerics.Vector3.Dot(n, hitNormal);
-                if (align < 0.95f) continue;   // within ~18° of the recorded normal
+                // |align|: the vertex-fan winding convention can flip the
+                // computed normal vs the physics plane's true facing — accept
+                // both signs (2026-07-06 sweep flaw fix).
+                if (MathF.Abs(align) < 0.95f) continue;   // within ~18° of the recorded normal
 
                 // Plane distance from the hit point.
                 float d = -System.Numerics.Vector3.Dot(n, w0);
@@ -358,4 +362,67 @@ public class Issue137CorridorSeamInspectionTests
         }
         _out.WriteLine("(downward sweep complete)");
     }
+
+    /// <summary>
+    /// 2026-07-06 gate-session follow-up: seam crossings SUCCEED at
+    /// y≈−40.8..−41.2 and BLOCK at y≈−39.5..−39.8 (cell-transit log,
+    /// launch-137-corridor-gate.log). A y-dependent boundary with no physics
+    /// polygon culprit points at the PORTAL POLYGONS — if the doorway
+    /// openings don't span the full corridor width, the transit/membership
+    /// machinery only hands the sphere to the neighbor inside the portal
+    /// poly's span. Dump every portal polygon's world-space vertex extent.
+    /// </summary>
+    [Theory]
+    [InlineData(0x8A02016Eu)]
+    [InlineData(0x8A02017Au)]
+    public void CorridorCell_PortalPolygonWorldSpans(uint envCellId)
+    {
+        var datDir = Env.GetEnvironmentVariable("ACDREAM_DAT_DIR")
+            ?? Path.Combine(Env.GetFolderPath(Env.SpecialFolder.UserProfile),
+                            "Documents", "Asheron's Call");
+        if (!Directory.Exists(datDir))
+        {
+            _out.WriteLine($"SKIP: dat directory not found at {datDir}");
+            return;
+        }
+
+        using var dats = new DatCollection(datDir, DatAccessType.Read);
+
+        var envCell = dats.Get<EnvCell>(envCellId);
+        Assert.NotNull(envCell);
+        var environment = dats.Get<DatReaderWriter.DBObjs.Environment>(0x0D000000u | envCell!.EnvironmentId);
+        Assert.NotNull(environment);
+        Assert.True(environment!.Cells.TryGetValue(envCell.CellStructure, out var cs));
+
+        var rot = new System.Numerics.Quaternion(
+            envCell.Position.Orientation.X, envCell.Position.Orientation.Y,
+            envCell.Position.Orientation.Z, envCell.Position.Orientation.W);
+        var world = System.Numerics.Matrix4x4.CreateFromQuaternion(rot)
+                  * System.Numerics.Matrix4x4.CreateTranslation(
+                        envCell.Position.Origin.X, envCell.Position.Origin.Y, envCell.Position.Origin.Z);
+
+        _out.WriteLine($"=== 0x{envCellId:X8} portal polygons (world spans) ===");
+        foreach (var p in envCell.CellPortals)
+        {
+            if (!cs!.Polygons.TryGetValue((ushort)p.PolygonId, out var poly))
+            {
+                _out.WriteLine($"  portal poly {p.PolygonId} -> 0x{p.OtherCellId:X4} {p.Flags}: NOT in visual set");
+                continue;
+            }
+
+            var min = new System.Numerics.Vector3(float.MaxValue);
+            var max = new System.Numerics.Vector3(float.MinValue);
+            foreach (var vid in poly.VertexIds)
+            {
+                if (!cs.VertexArray.Vertices.TryGetValue((ushort)vid, out var v)) continue;
+                var w = System.Numerics.Vector3.Transform(v.Origin, world);
+                min = System.Numerics.Vector3.Min(min, w);
+                max = System.Numerics.Vector3.Max(max, w);
+            }
+            _out.WriteLine(
+                $"  portal poly {p.PolygonId} -> 0x{p.OtherCellId:X4} [{p.Flags}] " +
+                $"x=[{min.X:F2},{max.X:F2}] y=[{min.Y:F2},{max.Y:F2}] z=[{min.Z:F2},{max.Z:F2}] " +
+                $"verts={poly.VertexIds.Count}");
+        }
+    }
 }

tests/AcDream.Core.Tests/Physics/Issue137CorridorSeamReplayTests.cs

@@ -73,6 +73,22 @@ public class Issue137CorridorSeamReplayTests
                 toLoad.Add(0x8A020000u | p.OtherCellId);
         }
 
+        // Expand three portal rings — the live collision cell array reaches
+        // cells three hops out (0x8A020166, the under-ramp room whose ceiling
+        // is the ramp slab's underside, is ring-3 in the 2026-07-06
+        // seam-shake trace; with only two rings the offline flood can never
+        // add it and the shake does not reproduce).
+        for (int ring = 0; ring < 3; ring++)
+        {
+            foreach (var known in new System.Collections.Generic.List<uint>(toLoad))
+            {
+                var cell = dats.Get<EnvCell>(known);
+                if (cell is null) continue;
+                foreach (var p in cell.CellPortals)
+                    toLoad.Add(0x8A020000u | p.OtherCellId);
+            }
+        }
+
         foreach (var cellId in toLoad)
         {
             var envCell = dats.Get<EnvCell>(cellId);
@@ -131,7 +147,145 @@ public class Issue137CorridorSeamReplayTests
             body:           body,
             moverFlags:     ObjectInfoState.IsPlayer | ObjectInfoState.EdgeSlide);
 
-    [Fact]
+    /// <summary>
+    /// 2026-07-06 seam-shake repro, snapshot-exact (probe session
+    /// launch-137-seam-probes.log + resolve-137-seam-capture.jsonl tick 4101,
+    /// repeated ×46): running WEST across the x=75 boundary
+    /// (0x8A02016E → 0x8A020165, the ramp cell) from (75.287, −40.035, −6)
+    /// toward (74.685, −39.988, −6), the resolve blocks with the SYNTHETIC
+    /// reversed-movement normal (0.997, −0.078, −0.002) and out==in — every
+    /// frame — the "shaking at the seam" report.
+    ///
+    /// <para>
+    /// Probe-traced chain: the foot sphere (tangent to the floor) crosses
+    /// onto 0165's ramp floor; the ramp slab is double-faced and the
+    /// UNDERSIDE face (poly 0, n=(−0.03,0,−1)) grazes the sphere within the
+    /// hit threshold → recorded as a foot near-miss → neg-poly step-up
+    /// dispatch with a downward normal → the nested step-up's walkable probe
+    /// rejects the exactly-tangent ramp floor ([walkable-nearest]
+    /// gap=−0.0000 overlapsSphere=False) → StepUpSlide →
+    /// slide_sphere(downward normal vs up-facing contact plane) → the
+    /// opposing-normals branch → Collided → revert. Repeat.
+    /// </para>
+    /// </summary>
+    [Fact(Skip = "#137 seam shake — deterministic RED repro; the post-step-up check-pos restore clobbers the lifted position (P2 stale-snapshot class). Un-skip with the fix.")]
+    public void SeamShake_WestBoundary_SnapshotExact_Advances()
+    {
+        var datDir = FindDatDir();
+        if (datDir is null)
+        {
+            _out.WriteLine("SKIP: dat directory not found");
+            return;
+        }
+
+        using var dats = new DatCollection(datDir, DatAccessType.Read);
+        var engine = BuildCorridorEngine(dats);
+
+        // Body seeded EXACTLY from the capture's bodyBefore (tick 4101).
+        var body = new PhysicsBody();
+        body.ContactPlaneValid   = true;
+        body.ContactPlane        = new Plane(Vector3.UnitZ, 6f);
+        body.ContactPlaneCellId  = SeamCellWest;
+        body.TransientState     |= TransientStateFlags.Contact | TransientStateFlags.OnWalkable;
+        body.WalkablePolygonValid = true;
+        body.WalkablePlane        = new Plane(Vector3.UnitZ, 6f);
+        body.WalkableUp           = Vector3.UnitZ;
+        body.WalkableVertices     = new[]
+        {
+            new Vector3(75f,       -38.33333f, -6f),
+            new Vector3(75f,       -41.66667f, -6f),
+            new Vector3(78.33333f, -41.66667f, -6f),
+            new Vector3(78.33333f, -38.33333f, -6f),
+        };
+
+        var from = new Vector3(75.28674f, -40.03537f, -6f);
+        var to   = new Vector3(74.6854f,  -39.988018f, -6f);
+
+        // Emit the same step-level probes the live session logged so the
+        // offline trace can be line-diffed against launch-137-seam-probes.log
+        // — the first divergent line names the state the replay is missing.
+        var probeBuffer = new System.IO.StringWriter();
+        var prevOut = Console.Out;
+        ResolveResult r1;
+        try
+        {
+            Console.SetOut(probeBuffer);
+            PhysicsDiagnostics.ProbeStepWalkEnabled  = true;
+            PhysicsDiagnostics.ProbePushBackEnabled  = true;
+            PhysicsDiagnostics.ProbeIndoorBspEnabled = true;
+
+            r1 = engine.ResolveWithTransition(
+                currentPos:     from,
+                targetPos:      to,
+                cellId:         SeamCellWest,
+                sphereRadius:   0.48f,
+                sphereHeight:   1.2f,
+                stepUpHeight:   0.6f,    // live Setup values from the capture
+                stepDownHeight: 1.5f,
+                isOnGround:     true,
+                body:           body,
+                moverFlags:     ObjectInfoState.IsPlayer | ObjectInfoState.EdgeSlide);
+        }
+        finally
+        {
+            PhysicsDiagnostics.ProbeStepWalkEnabled  = false;
+            PhysicsDiagnostics.ProbePushBackEnabled  = false;
+            PhysicsDiagnostics.ProbeIndoorBspEnabled = false;
+            Console.SetOut(prevOut);
+        }
+        _out.WriteLine(probeBuffer.ToString());
+
+        _out.WriteLine($"r1: ok={r1.Ok} out=({r1.Position.X:F3},{r1.Position.Y:F3},{r1.Position.Z:F3}) " +
+                       $"cell=0x{r1.CellId:X8} hit={r1.CollisionNormalValid} " +
+                       $"n=({r1.CollisionNormal.X:F2},{r1.CollisionNormal.Y:F2},{r1.CollisionNormal.Z:F2}) " +
+                       $"bodySliding={body.TransientState.HasFlag(TransientStateFlags.Sliding)}");
+
+        Assert.True(r1.Position.X < from.X - 0.3f,
+            $"The westward boundary crossing onto the ramp must advance " +
+            $"({from.X:F3} → {r1.Position.X:F3}, target {to.X:F3}); zero " +
+            $"advance with the reversed-movement normal = the seam shake.");
+    }
+
+    /// <summary>
+    /// 2026-07-06 gate session repro (launch-137-corridor-gate.log): standing
+    /// at (84.851, −39.764, −6.000) — the foot sphere already straddling the
+    /// x=85 cell boundary by 0.33 m — the first move attempt toward
+    /// (85.453, −39.782) blocked with the synthetic reversed-movement normal
+    /// (−1.00, 0.03, −0.02), out==in, cp lost (cp=none), and repeated every
+    /// frame (the "shaking at the seam" report). The deeper straddle start is
+    /// what the original replay frame (84.638 → 85.253) didn't cover.
+    /// </summary>
+    [Fact(Skip = "#137 seam shake — deterministic RED repro; the post-step-up check-pos restore clobbers the lifted position (P2 stale-snapshot class). Un-skip with the fix.")]
+    public void SeamCrossing_FromDeepStraddleStart_Advances()
+    {
+        var datDir = FindDatDir();
+        if (datDir is null)
+        {
+            _out.WriteLine("SKIP: dat directory not found");
+            return;
+        }
+
+        using var dats = new DatCollection(datDir, DatAccessType.Read);
+        var engine = BuildCorridorEngine(dats);
+        var body   = GroundedBody();
+
+        var from = new Vector3(84.851f, -39.764f, -6.000f);
+        var to   = new Vector3(85.453f, -39.782f, -6.000f);
+
+        var r1 = Resolve(engine, body, from, to, SeamCellWest);
+        _out.WriteLine($"r1: ok={r1.Ok} out=({r1.Position.X:F3},{r1.Position.Y:F3},{r1.Position.Z:F3}) " +
+                       $"cell=0x{r1.CellId:X8} hit={r1.CollisionNormalValid} " +
+                       $"n=({r1.CollisionNormal.X:F2},{r1.CollisionNormal.Y:F2},{r1.CollisionNormal.Z:F2}) " +
+                       $"bodySliding={body.TransientState.HasFlag(TransientStateFlags.Sliding)} " +
+                       $"bodyCpValid={body.ContactPlaneValid}");
+
+        Assert.True(r1.Position.X > from.X + 0.2f,
+            $"The straddling-start seam crossing must advance " +
+            $"({from.X:F3} → {r1.Position.X:F3}); zero advance with a " +
+            $"reversed-movement normal = the 2026-07-06 seam shake.");
+    }
+
+    [Fact(Skip = "#137 seam shake — deterministic RED repro; the post-step-up check-pos restore clobbers the lifted position (P2 stale-snapshot class). Un-skip with the fix.")]
     public void SeamCrossing_DoesNotPersistSyntheticSlidingNormal_AndRunContinues()
     {
         var datDir = FindDatDir();