diff --git a/src/AcDream.App/Rendering/GameWindow.cs b/src/AcDream.App/Rendering/GameWindow.cs
index 8cc0ce6..79f79c9 100644
--- a/src/AcDream.App/Rendering/GameWindow.cs
+++ b/src/AcDream.App/Rendering/GameWindow.cs
@@ -7139,7 +7139,21 @@ public sealed class GameWindow : IDisposable
             // Step 4: portal visibility — compute BEFORE the UBO upload so
             // the indoor flag drives the sun's intensity to zero for
             // dungeons (r13 §13.7).
-            var visibility = _cellVisibility.ComputeVisibility(camPos);
+            // Phase U.4c (2026-05-31): root indoor visibility at the PLAYER's cell, not the
+            // camera EYE. Retail's CellManager::ChangePosition (0x004559B0) tracks curr_cell by
+            // the player/physics position. The 3rd-person chase EYE drifts out of the player's
+            // cell (through interior walls into AABB gaps); FindCameraCell then can't place the
+            // eye and returns the STALE previous cell for its 3 grace frames, from which the
+            // doorway portal is "behind" the eye → culled → the exit cell + terrain + shells
+            // flap off. ACDREAM_PROBE_FLAP capture (2026-05-31): every flap frame is
+            // res=Grace eyeInRoot=n terrain=Skip; every good frame is eyeInRoot=Y. The eye is
+            // still used for the per-frame PROJECTION (envCellViewProj) — only the cell ROOT +
+            // portal-side test track the player. This mirrors the playerInsideCell lighting
+            // decision below, which already roots at the player for exactly this reason.
+            var visRootPos = (_playerMode && _playerController is not null)
+                ? _playerController.Position
+                : camPos;
+            var visibility = _cellVisibility.ComputeVisibility(visRootPos);
             bool cameraInsideCell = visibility?.CameraCell is not null;
 
             // Phase U.4 (2026-05-30): the [vis] probe moved DOWN to the unified
@@ -7285,9 +7299,12 @@ public sealed class GameWindow : IDisposable
             HashSet<uint>? envCellShellFilter = null; // drawable visible cells (cellIdToSlot keys)
             if (clipRoot is not null)
             {
+                // Phase U.4c: side test + distance ordering use the PLAYER position (visRootPos,
+                // stable inside the cell); projection uses the eye's envCellViewProj (the screen
+                // view). See the visRootPos rationale at the ComputeVisibility call above.
                 var pvFrame = PortalVisibilityBuilder.Build(
                     clipRoot,
-                    camPos,
+                    visRootPos,
                     id => _cellVisibility.TryGetCell(id, out var c) ? c : null,
                     envCellViewProj);
 
diff --git a/tests/AcDream.App.Tests/Rendering/PortalVisibilityBuilderTests.cs b/tests/AcDream.App.Tests/Rendering/PortalVisibilityBuilderTests.cs
index 659add2..9d4fb02 100644
--- a/tests/AcDream.App.Tests/Rendering/PortalVisibilityBuilderTests.cs
+++ b/tests/AcDream.App.Tests/Rendering/PortalVisibilityBuilderTests.cs
@@ -365,59 +365,18 @@ public class PortalVisibilityBuilderTests
     }
 
     // -----------------------------------------------------------------------
-    // 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.
+    // Phase U.4c: an earlier synthetic flap test (Build_NearBoundaryIntermediatePortal)
+    // lived here. It modeled the doorway flap as a BUILDER side-test cull dropping the
+    // exit cell. The live ACDREAM_PROBE_FLAP capture (2026-05-31) DISPROVED that model:
+    // the real cause is the visibility ROOT being driven by the 3rd-person camera EYE —
+    // the eye drifts out of the player's cell, FindCameraCell returns a STALE cell for
+    // its grace frames, and the doorway is then culled as "behind" the eye. The fix is
+    // at the GameWindow integration level (root visibility at the PLAYER's cell:
+    // visRootPos), NOT in the builder — the builder's side test is correct and unchanged,
+    // so the old test asserted a non-bug and was removed rather than left red. The fix is
+    // validated by the visual gate + the [flap] probe (RenderingDiagnostics.ProbeFlapEnabled);
+    // see docs/research/2026-05-31-u4c-flap-characterization.md.
     // -----------------------------------------------------------------------
-    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)