fix(physics): #42 skip self in FindObjCollisions — airborne XY drift

Root cause confirmed via two-run diagnostic and the named-retail decomp:
the airborne sweep was colliding with the moving entity's OWN ShadowEntry
because FindObjCollisions had no self-skip filter. Live entities (local
player, remotes) register a Cylinder in ShadowObjectRegistry on spawn
(GameWindow.cs:2545) and UpdatePosition tracks its world position each
tick, so the moving sphere's own cylinder is always at the body's
position. Without a gate, CylinderCollision sees the sphere overlapping
its own cylinder volume and slides the sphere ~1m horizontally on every
frame the path produces non-zero motion.

Why grounded mostly hides it and airborne exposes it:
- Stationary grounded → numSteps=0, TransitionalInsert never runs.
- Walking grounded → push fires but motion escapes the cyl radius and
  the deflection blends into normal motion.
- Stationary airborne (jump) → pure +Z motion; the cyl push is the
  only horizontal contribution and manifests as a clean ~1m drift.

Run-2 evidence (launch-42-r2.log) — 152 [SWEEP-OBJ] events, every one
with type=Cylinder, gfxObj=0x02000001 (humanoid setup), R=0.679,
H=1.835, at obj.Position EXACTLY matching the body's pre.Position. Run
1 had already ruled out H1 (cpN=(0,0,1) flat, no slope projection).

Retail does the same skip — CObjCell::find_obj_collisions at
named-retail acclient_2013_pseudo_c.txt:308931:

    if ((physobj->parent == 0 && physobj != arg2->object_info.object))

`arg2->object_info.object` is the OBJECTINFO::object self-pointer set
by OBJECTINFO::init at acclient_2013_pseudo_c.txt:274435. Our port
mirrors this with an EntityId-based filter:

  - ObjectInfo gains a SelfEntityId field (default 0 = no filter).
  - ResolveWithTransition gains an optional `uint movingEntityId = 0`
    parameter that sets it.
  - FindObjCollisions skips entries whose EntityId matches
    SelfEntityId when the id is non-zero.
  - PlayerMovementController gains a LocalEntityId property; GameWindow
    refreshes it per-tick from `_entitiesByServerGuid[_playerServerGuid]`.
  - GameWindow's airborne-remote ResolveWithTransition call site passes
    `movingEntityId: kv.Key` (kv.Key is the local entity id keying
    `_animatedEntities`, same id used at the spawn-time
    ShadowObjects.Register).

Default 0 keeps tests and one-shot callers (no registered ShadowEntry)
working unchanged.

Lock-the-fix unit test:
`PhysicsEngineTests.ResolveWithTransition_SelfShadowEntry_NotPushedWhenIdMatches`
registers a humanoid Cylinder at the body's exact position (matching
GameWindow's spawn pattern), then asserts that:
  - movingEntityId=0 (control)        → unfiltered XY drift > 0.5m
  - movingEntityId=registered id (fix) → XY drift ≈ 0

Diagnostic wiring (a36369d + this commit's [SWEEP-OBJ] addition) stays
in tree, env-var gated (ACDREAM_AIRBORNE_DIAG=1) so it produces no
output in normal use but lets us verify the fix on the live client and
debug future regressions.

Build: green. Tests: 355 pass, 6 fail (all pre-existing per the handoff
prompt — verified by stashing this change; the BSPStepUp C3 failure is
on the prior commit too).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.App/Input/PlayerMovementController.cs

@@ -130,6 +130,17 @@ public sealed class PlayerMovementController
     public Vector3 Position => _body.Position;
     public uint CellId { get; private set; }
 
+    /// <summary>
+    /// Local-player entity id used to skip self-collision in the
+    /// airborne sweep. GameWindow updates this whenever the local
+    /// `+Acdream` entity (re)spawns. Default 0 = no filter (matches
+    /// retail's CObjCell::find_obj_collisions self-skip when the
+    /// caller's OBJECTINFO::object pointer is null). Without this the
+    /// sweep collides with its own ShadowEntry registered at
+    /// GameWindow.cs:2545 — see #42.
+    /// </summary>
+    public uint LocalEntityId { get; set; }
+
     public bool IsAirborne => !_body.OnWalkable;
 
     /// <summary>
@@ -558,7 +569,12 @@ public sealed class PlayerMovementController
             // through other non-PK players, which is retail's default for
             // ACE's character creation defaults too).
             moverFlags: AcDream.Core.Physics.ObjectInfoState.IsPlayer
-                      | AcDream.Core.Physics.ObjectInfoState.EdgeSlide);
+                      | AcDream.Core.Physics.ObjectInfoState.EdgeSlide,
+            // Fix #42: skip self in FindObjCollisions. Wired by GameWindow
+            // when the local player entity spawns (or stays 0 in tests, in
+            // which case there's no registered ShadowEntry to collide with
+            // anyway).
+            movingEntityId: LocalEntityId);
 
         // L.4-diag (2026-04-30): trace position transitions so we can see
         // whether the body is actually moving frame-to-frame on the steep

src/AcDream.App/Rendering/GameWindow.cs

@@ -5264,6 +5264,18 @@ public sealed class GameWindow : IDisposable
                 MouseDeltaX: 0f,
                 Jump:        _inputDispatcher.IsActionHeld(AcDream.UI.Abstractions.Input.InputAction.MovementJump));
 
+            // Fix #42 (2026-05-05): keep PlayerMovementController's
+            // LocalEntityId in sync with the live local player entity so
+            // FindObjCollisions skips its own ShadowEntry. Re-fetched per
+            // tick so re-spawns / character switches don't leave a stale
+            // id on the controller. Pre-spawn or between-character it
+            // stays 0 (no filter), which is harmless because there's no
+            // ShadowEntry registered yet.
+            _playerController.LocalEntityId =
+                _entitiesByServerGuid.TryGetValue(_playerServerGuid, out var localEnt)
+                    ? localEnt.Id
+                    : 0u;
+
             var result = _playerController.Update((float)dt, input);
 
             // Update the player entity's position + rotation so it renders at
@@ -6487,7 +6499,16 @@ public sealed class GameWindow : IDisposable
                             // Retail default physics state includes EdgeSlide.
                             // Remote dead-reckoning should exercise the same
                             // edge/cliff branch as local movement.
-                            moverFlags: AcDream.Core.Physics.ObjectInfoState.EdgeSlide);
+                            moverFlags: AcDream.Core.Physics.ObjectInfoState.EdgeSlide,
+                            // Fix #42 (2026-05-05): skip the moving remote's
+                            // own ShadowEntry. _animatedEntities is keyed by
+                            // entity.Id so kv.Key matches the EntityId the
+                            // ShadowObjectRegistry has for this remote.
+                            // Without this, the airborne sweep collides with
+                            // the remote's own cylinder and produces ~1m of
+                            // horizontal drift on the first jump frame
+                            // (validated by [SWEEP-OBJ] traces).
+                            movingEntityId: kv.Key);
 
                         rm.Body.Position = resolveResult.Position;
                         if (resolveResult.CellId != 0)

src/AcDream.Core/Physics/PhysicsEngine.cs

@@ -473,12 +473,22 @@ public sealed class PhysicsEngine
         float stepUpHeight, float stepDownHeight,
         bool isOnGround,
         PhysicsBody? body = null,
-        ObjectInfoState moverFlags = ObjectInfoState.None)
+        ObjectInfoState moverFlags = ObjectInfoState.None,
+        uint movingEntityId = 0)
     {
         var transition = new Transition();
         transition.ObjectInfo.StepUpHeight = stepUpHeight;
         transition.ObjectInfo.StepDownHeight = stepDownHeight;
         transition.ObjectInfo.StepDown = true;
+        // Fix #42 (2026-05-05): the moving entity's ShadowEntry must be
+        // skipped in FindObjCollisions or the sweep collides with self.
+        // Default 0 keeps tests / one-shot callers (no registered entity)
+        // working. Plumbed through ObjectInfo because retail stores the
+        // self pointer on OBJECTINFO::object (named-retail
+        // acclient_2013_pseudo_c.txt:274435 OBJECTINFO::init →
+        // this->object = arg2). The skip itself is at
+        // CObjCell::find_obj_collisions line 308931.
+        transition.ObjectInfo.SelfEntityId = movingEntityId;
 
         // Commit C 2026-04-29 — caller-supplied mover flags drive the
         // retail PvP exemption block in FindObjCollisions. The local

src/AcDream.Core/Physics/TransitionTypes.cs

@@ -56,6 +56,19 @@ public sealed class ObjectInfo
     public bool StepDown = true;
     public float Scale = 1.0f;
 
+    /// <summary>
+    /// EntityId of the moving entity, used by <c>FindObjCollisions</c> to
+    /// skip the moving object's own ShadowEntry. Mirrors retail
+    /// <c>CObjCell::find_obj_collisions</c> at named-retail line 308931
+    /// (<c>physobj != arg2->object_info.object</c>) — the self pointer
+    /// stored on <c>OBJECTINFO::object</c>. Default 0 = no filter (tests
+    /// and one-shot callers that don't have a registered entity).
+    /// Without this gate the local player and airborne remotes collide
+    /// with their own registered cylinder in <c>ShadowObjectRegistry</c>,
+    /// producing the ~1m horizontal push-out documented in #42.
+    /// </summary>
+    public uint SelfEntityId;
+
     // Convenience flag checks
     public bool Contact     => State.HasFlag(ObjectInfoState.Contact);
     public bool OnWalkable  => State.HasFlag(ObjectInfoState.OnWalkable);
@@ -1375,6 +1388,19 @@ public sealed class Transition
         if (engine.DataCache is null) return TransitionState.OK;
 
         var sp = SpherePath;
+        var oi = ObjectInfo;
+
+        // #42 diagnostic (2026-05-05): identify which static object causes
+        // the airborne first-frame ~1m push. Capture sphere check pos at
+        // entry; on a non-OK return, we'll log the (object, delta) pair
+        // gated on ACDREAM_AIRBORNE_DIAG=1 + airborne. The first evidence
+        // run ruled out H1 (slope-driven AdjustOffset projection); cpN was
+        // (0,0,1) flat for every drift event, so the horizontal push must
+        // come from CylinderCollision or BSPQuery.FindCollisions inside
+        // this function. Logging the object identity tells us which one.
+        bool airborneDiag = !oi.Contact
+            && Environment.GetEnvironmentVariable("ACDREAM_AIRBORNE_DIAG") == "1";
+        Vector3 sphereCheckBefore = sp.CheckPos;
 
         Vector3 checkPos     = sp.GlobalSphere[0].Origin;
         Vector3 currPos      = sp.GlobalCurrCenter[0].Origin;
@@ -1397,6 +1423,21 @@ public sealed class Transition
 
         foreach (var obj in nearbyObjs)
         {
+            // Self-skip — fix #42 (2026-05-05). Mirrors retail
+            // CObjCell::find_obj_collisions at acclient_2013_pseudo_c.txt
+            // 308931: `physobj != arg2->object_info.object` rejects the
+            // moving entity's own shadow entry. Live entities (creatures,
+            // players) register a Cylinder in ShadowObjectRegistry at
+            // GameWindow.cs:2545 which UpdatePosition tracks live, so
+            // without this skip the moving sphere collides with itself
+            // every frame the path produces non-zero motion. The grounded
+            // case mostly hides behind motion that escapes the radius;
+            // airborne stationary jumps expose it as a one-shot ~1m
+            // horizontal push (validated by [SWEEP-OBJ] traces with
+            // gfxObj=0x02000001 at exactly the entity's own position).
+            if (oi.SelfEntityId != 0 && obj.EntityId == oi.SelfEntityId)
+                continue;
+
             // Broad-phase: can the moving sphere reach this object?
             Vector3 deltaToCurr = currPos - obj.Position;
             float distToCurr;
@@ -1483,8 +1524,20 @@ public sealed class Transition
             }
 
             if (result != TransitionState.OK)
+            {
+                if (airborneDiag)
+                {
+                    var sphereCheckAfter = sp.CheckPos;
+                    var d = sphereCheckAfter - sphereCheckBefore;
+                    Console.WriteLine(
+                        $"[SWEEP-OBJ] type={obj.CollisionType} gfxObj=0x{obj.GfxObjId:X8} " +
+                        $"objPos=({obj.Position.X:F3},{obj.Position.Y:F3},{obj.Position.Z:F3}) " +
+                        $"objR={obj.Radius:F3} cylH={obj.CylHeight:F3} " +
+                        $"state={result} pushDelta=({d.X:F3},{d.Y:F3},{d.Z:F3})");
+                }
                 return result;
             }
+        }
 
         return TransitionState.OK;
     }

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

@@ -377,4 +377,80 @@ public class PhysicsEngineTests
 
         Assert.False(result.IsOnGround);
     }
+
+    /// <summary>
+    /// #42 lock — when the moving entity's own ShadowEntry is registered
+    /// in <see cref="ShadowObjectRegistry"/> at the body's exact position
+    /// (the production pattern from <c>GameWindow.cs:2545</c> spawn → register
+    /// + <c>UpdatePosition</c> live tracking), the airborne sweep MUST skip
+    /// it. Without the gate, <c>FindObjCollisions</c> sees the cylinder as
+    /// a foreign collidable and slides the sphere ~1m horizontally on the
+    /// first non-zero-motion frame — the bug observed by the [SWEEP-OBJ]
+    /// trace and reported as the post-jump XY drift in #42.
+    /// <para>
+    /// Mirrors retail's self-skip at <c>CObjCell::find_obj_collisions</c>
+    /// (named-retail <c>acclient_2013_pseudo_c.txt:308931</c>):
+    /// <c>physobj != arg2->object_info.object</c>.
+    /// </para>
+    /// </summary>
+    [Fact]
+    public void ResolveWithTransition_SelfShadowEntry_NotPushedWhenIdMatches()
+    {
+        var engine = MakeFlatEngine(terrainZ: 50f);
+        // FindObjCollisions early-returns when DataCache is null. An empty
+        // cache is enough for cylinder objects; only BSP objects look up
+        // entries inside.
+        engine.DataCache = new PhysicsDataCache();
+
+        const uint movingEntityId = 0xDEADBEEFu;
+        var bodyPos   = new Vector3(96f, 96f, 50f);
+        var targetPos = bodyPos + new Vector3(0f, 0f, 0.022f);   // stationary +Z
+
+        // Register the moving entity's own ShadowEntry — humanoid Cylinder
+        // sized to match the live-spawn registration in production
+        // (GameWindow.cs:2545). The gfxObj id 0x02000001 is the standard
+        // human setup; radius/height match the [SWEEP-OBJ] trace observed
+        // during run #2 of the #42 investigation.
+        engine.ShadowObjects.Register(
+            entityId:      movingEntityId,
+            gfxObjId:      0x02000001u,
+            worldPos:      bodyPos,
+            rotation:      Quaternion.Identity,
+            radius:        0.679f,
+            worldOffsetX:  0f, worldOffsetY: 0f,
+            landblockId:   0xA9B4FFFFu,
+            collisionType: ShadowCollisionType.Cylinder,
+            cylHeight:     1.835f);
+
+        // Without the gate (movingEntityId == 0): the sweep must self-push.
+        // This proves the registry actually causes a collision, so the
+        // following filtered case is not a vacuous pass.
+        var unfiltered = engine.ResolveWithTransition(
+            currentPos: bodyPos, targetPos: targetPos,
+            cellId: 0xA9B40039u,
+            sphereRadius: 0.48f, sphereHeight: 1.2f,
+            stepUpHeight: 0.4f, stepDownHeight: 0.4f,
+            isOnGround: false,
+            movingEntityId: 0u);
+
+        float unfilteredXY = MathF.Sqrt(
+            (unfiltered.Position.X - targetPos.X) * (unfiltered.Position.X - targetPos.X) +
+            (unfiltered.Position.Y - targetPos.Y) * (unfiltered.Position.Y - targetPos.Y));
+        Assert.True(unfilteredXY > 0.5f,
+            $"Without movingEntityId, sweep should self-push (got XY drift {unfilteredXY:F3}m)");
+
+        // With the gate: the sweep must leave XY unchanged.
+        var filtered = engine.ResolveWithTransition(
+            currentPos: bodyPos, targetPos: targetPos,
+            cellId: 0xA9B40039u,
+            sphereRadius: 0.48f, sphereHeight: 1.2f,
+            stepUpHeight: 0.4f, stepDownHeight: 0.4f,
+            isOnGround: false,
+            movingEntityId: movingEntityId);
+
+        float filteredXY = MathF.Sqrt(
+            (filtered.Position.X - targetPos.X) * (filtered.Position.X - targetPos.X) +
+            (filtered.Position.Y - targetPos.Y) * (filtered.Position.Y - targetPos.Y));
+        Assert.InRange(filteredXY, 0f, 0.001f);
+    }
 }