diff --git a/tests/AcDream.App.Tests/Rendering/PortalVisibilityBuilderTests.cs b/tests/AcDream.App.Tests/Rendering/PortalVisibilityBuilderTests.cs
index a9fb2c9..659add2 100644
--- a/tests/AcDream.App.Tests/Rendering/PortalVisibilityBuilderTests.cs
+++ b/tests/AcDream.App.Tests/Rendering/PortalVisibilityBuilderTests.cs
@@ -364,6 +364,61 @@ public class PortalVisibilityBuilderTests
             $"neighbour CellView MinX {bView.MinX} must still cover the LEFT reciprocal opening (minX {leftMinX})");
     }
 
+    // -----------------------------------------------------------------------
+    // Phase U.4c: the threshold "flap". A chain camera(C0) -> mid(C1) -> exit(C2)
+    // where the C0->C1 portal's clip plane sits just in front of the camera.
+    // BOTH poses are legitimately inside C0 and SHOULD see the exit window; they
+    // straddle the C0->C1 side-test boundary by a few cm. Pre-fix, the pose just
+    // behind the plane hard-culls C0->C1 (CameraOnInteriorSide), C2 is never
+    // reached, and OutsideView empties — the flap. The fix must keep the exit
+    // cell visible (OutsideView non-empty) at BOTH poses.
+    // -----------------------------------------------------------------------
+    private static Matrix4x4 ViewProjAt(Vector3 eye)
+    {
+        var view = Matrix4x4.CreateLookAt(eye, eye + new Vector3(0, 0, -1), Vector3.UnitY);
+        var proj = Matrix4x4.CreatePerspectiveFieldOfView(1.2f, 1.0f, 0.1f, 1000f);
+        return view * proj;
+    }
+
+    private static (LoadedCell cam, Dictionary<uint, LoadedCell> all) FlapChain()
+    {
+        const uint C0 = 0x0001, C1 = 0x0002, C2 = 0x0003;
+        // C0 -> C1 portal at z=-1, with a clip plane (normal +Z, InsideSide=0) at z=-1.
+        // dot = camZ + D; with D = 1 the plane is at camZ = -1: inside iff camZ >= -1 - eps.
+        var c0 = Cell(C0, new CellPortalInfo((ushort)C1, 0, 0, 0));
+        c0.PortalPolygons.Add(Quad(0f, 0f, 0.6f, 0.6f, -1f));
+        c0.ClipPlanes.Add(new PortalClipPlane { Normal = new Vector3(0, 0, 1), D = 1f, InsideSide = 0 });
+        // C1 -> C2 (no clip plane → never culled), C2 has the exit window.
+        var c1 = Cell(C1, new CellPortalInfo((ushort)C2, 0, 0, 0));
+        c1.PortalPolygons.Add(Quad(0f, 0f, 0.6f, 0.6f, -4f));
+        var c2 = Cell(C2, new CellPortalInfo(0xFFFF, 0, 0, 0));
+        c2.PortalPolygons.Add(Quad(0f, 0f, 1.0f, 1.0f, -7f));
+        var all = new Dictionary<uint, LoadedCell> { [C0] = c0, [C1] = c1, [C2] = c2 };
+        return (c0, all);
+    }
+
+    [Fact]
+    public void Build_NearBoundaryIntermediatePortal_ExitCellStaysVisibleAcrossPose()
+    {
+        var (cam, all) = FlapChain();
+        Func<uint, LoadedCell?> lookup = id => all.TryGetValue(id, out var c) ? c : null;
+
+        // Pose A: a few cm IN FRONT of the C0->C1 plane (camZ = -0.9 >= -1 → inside).
+        var poseA = new Vector3(0, 0, -0.9f);
+        var frameA = PortalVisibilityBuilder.Build(cam, poseA, lookup, ViewProjAt(poseA));
+
+        // Pose B: a few cm BEHIND it (camZ = -1.1 < -1 → pre-fix the side test culls C0->C1).
+        var poseB = new Vector3(0, 0, -1.1f);
+        var frameB = PortalVisibilityBuilder.Build(cam, poseB, lookup, ViewProjAt(poseB));
+
+        // The exit cell — and therefore OutsideView — must be present at BOTH poses.
+        Assert.False(frameA.OutsideView.IsEmpty, "pose A should see the exit window");
+        Assert.False(frameB.OutsideView.IsEmpty,
+            "pose B (a few cm away) must ALSO see the exit window — this is the flap: " +
+            "an intermediate side-test flip must not drop the exit cell from the set");
+        Assert.Contains(0x0003u, frameB.OrderedVisibleCells);
+    }
+
     // Signed-area-magnitude (shoelace) sum over a CellView's polygons in NDC.
     private static float CellViewArea(CellView view)
     {