diff --git a/src/AcDream.App/Rendering/RetailChaseCamera.cs b/src/AcDream.App/Rendering/RetailChaseCamera.cs
index 993999a..d05b907 100644
--- a/src/AcDream.App/Rendering/RetailChaseCamera.cs
+++ b/src/AcDream.App/Rendering/RetailChaseCamera.cs
@@ -27,6 +27,14 @@ public sealed class RetailChaseCamera : ICamera
 {
     // ICamera surface.
     public Vector3   Position   { get; private set; }
+
+    /// <summary>
+    /// The cell the collided viewer-sphere ended in (retail <c>viewer_cell =
+    /// sphere_path.curr_cell</c>). Roots the render mode + indoor visibility + the portal
+    /// side-test in <see cref="GameWindow"/> (Phase W single-viewpoint V1) — the ONE viewpoint.
+    /// Equals the passed player cell when camera collision is off / the probe is null.
+    /// </summary>
+    public uint      ViewerCellId { get; private set; }
     public float     Aspect     { get; set; } = 16f / 9f;
     public float     FovY       { get; set; } = MathF.PI / 3f;
     public Matrix4x4 View       { get; private set; } = Matrix4x4.Identity;
@@ -152,8 +160,16 @@ public sealed class RetailChaseCamera : ICamera
         //     eye is pressed against a wall. So collide into a separate local and
         //     leave _dampedEye as the clean, uncollided sought position.
         Vector3 publishedEye = _dampedEye;
+        // The viewer cell defaults to the player cell (collision off / null probe); the sweep
+        // overwrites it with the swept cell (retail viewer_cell). Always set so GameWindow has a
+        // robust per-frame "which cell is the camera in?" answer.
+        ViewerCellId = cellId;
         if (CameraDiagnostics.CollideCamera && CollisionProbe is not null)
-            publishedEye = CollisionProbe.SweepEye(pivotWorld, _dampedEye, cellId, selfEntityId).Eye;
+        {
+            var swept = CollisionProbe.SweepEye(pivotWorld, _dampedEye, cellId, selfEntityId);
+            publishedEye = swept.Eye;
+            ViewerCellId = swept.ViewerCellId;
+        }
 
         // 6. Publish renderer surface (from the collided eye; rotation stays the
         //    smoothly-damped look direction toward the pivot).
diff --git a/tests/AcDream.App.Tests/Rendering/RetailChaseCameraTests.cs b/tests/AcDream.App.Tests/Rendering/RetailChaseCameraTests.cs
index 0e58afb..989dcb2 100644
--- a/tests/AcDream.App.Tests/Rendering/RetailChaseCameraTests.cs
+++ b/tests/AcDream.App.Tests/Rendering/RetailChaseCameraTests.cs
@@ -477,6 +477,38 @@ public class RetailChaseCameraTests
         Assert.Equal(collided, cam.Position);
     }
 
+    [Fact]
+    public void Update_WithProbeAndFlagOn_ExposesSweptViewerCell()
+    {
+        CameraDiagnostics.CollideCamera = true;
+        var probe = new FakeProbe { ReturnEye = new Vector3(1f, 2f, 3f), ReturnCell = 0xA9B40170u };
+        var cam = new RetailChaseCamera { CollisionProbe = probe };
+
+        cam.Update(
+            playerPosition: Vector3.Zero, playerYaw: 0f, playerVelocity: Vector3.Zero,
+            isOnGround: true, contactPlaneNormal: Vector3.UnitZ, dt: 1f / 60f,
+            cellId: 0xA9B40171u, selfEntityId: 0x5);
+
+        Assert.Equal(0xA9B40170u, cam.ViewerCellId);   // the swept cell, not the player cell
+    }
+
+    [Fact]
+    public void Update_FlagOff_ViewerCellFallsBackToPlayerCell()
+    {
+        CameraDiagnostics.CollideCamera = false;
+        try
+        {
+            var cam = new RetailChaseCamera { CollisionProbe = new FakeProbe { ReturnCell = 0xDEADu } };
+            cam.Update(
+                playerPosition: Vector3.Zero, playerYaw: 0f, playerVelocity: Vector3.Zero,
+                isOnGround: true, contactPlaneNormal: Vector3.UnitZ, dt: 1f / 60f,
+                cellId: 0xA9B40171u, selfEntityId: 0x5);
+
+            Assert.Equal(0xA9B40171u, cam.ViewerCellId);   // collision off → the passed player cell
+        }
+        finally { CameraDiagnostics.CollideCamera = true; }
+    }
+
     [Fact]
     public void Update_FlagOff_DoesNotConsultProbe()
     {