diff --git a/src/AcDream.App/Rendering/CellVisibility.cs b/src/AcDream.App/Rendering/CellVisibility.cs
index 84638cf..7dde694 100644
--- a/src/AcDream.App/Rendering/CellVisibility.cs
+++ b/src/AcDream.App/Rendering/CellVisibility.cs
@@ -142,6 +142,25 @@ public struct PortalClipPlane
     public int InsideSide;
 }
 
+/// <summary>
+/// Phase U.4c flap probe (diagnostic): which branch of
+/// <see cref="CellVisibility.FindCameraCell"/> resolved the camera cell.
+/// </summary>
+public enum CameraCellResolution
+{
+    /// <summary>No cell contains the eye (outdoors), or not yet resolved.</summary>
+    None,
+    /// <summary>The eye is inside the previously-cached cell (fast path).</summary>
+    Cache,
+    /// <summary>The eye is inside a one-hop portal neighbour of the cached cell.</summary>
+    Neighbour,
+    /// <summary>The eye is inside a cell found by the full brute-force scan.</summary>
+    BruteForce,
+    /// <summary>The eye is inside NO cell, but the previous cell is kept alive for a
+    /// few grace frames — the "stale root" case the flap probe watches for.</summary>
+    Grace,
+}
+
 /// <summary>
 /// Result of a portal-based visibility BFS from the camera cell.
 /// </summary>
@@ -213,6 +232,14 @@ public sealed class CellVisibility
     /// <summary>The last visibility result produced by <see cref="ComputeVisibility"/>.</summary>
     public VisibilityResult? LastVisibilityResult { get; private set; }
 
+    /// <summary>
+    /// Phase U.4c flap probe (diagnostic): which <see cref="FindCameraCell"/> branch
+    /// resolved the camera cell on the most recent call. A <see cref="CameraCellResolution.Grace"/>
+    /// (or <see cref="CameraCellResolution.Cache"/>) result while the eye is NOT actually inside the
+    /// returned cell is the "stale root" signature the flap probe looks for.
+    /// </summary>
+    public CameraCellResolution LastCameraCellResolution { get; private set; } = CameraCellResolution.None;
+
     // ------------------------------------------------------------------
     // Registration
     // ------------------------------------------------------------------
@@ -330,7 +357,10 @@ public sealed class CellVisibility
     {
         // 1. Fast path: cached cell.
         if (_lastCameraCell != null && PointInCell(cameraPos, _lastCameraCell))
+        {
+            LastCameraCellResolution = CameraCellResolution.Cache;
             return _lastCameraCell;
+        }
 
         // 2. One-hop neighbours of the cached cell.
         if (_lastCameraCell != null)
@@ -347,6 +377,7 @@ public sealed class CellVisibility
                 {
                     _lastCameraCell = neighbour;
                     _cellSwitchGraceFrames = CellSwitchGraceFrameCount;
+                    LastCameraCellResolution = CameraCellResolution.Neighbour;
                     return neighbour;
                 }
             }
@@ -361,6 +392,7 @@ public sealed class CellVisibility
                 {
                     _lastCameraCell = cell;
                     _cellSwitchGraceFrames = CellSwitchGraceFrameCount;
+                    LastCameraCellResolution = CameraCellResolution.BruteForce;
                     return cell;
                 }
             }
@@ -370,11 +402,13 @@ public sealed class CellVisibility
         if (_lastCameraCell != null && _cellSwitchGraceFrames > 0)
         {
             _cellSwitchGraceFrames--;
+            LastCameraCellResolution = CameraCellResolution.Grace;
             return _lastCameraCell;
         }
 
         // 5. Camera is outside all cells.
         _lastCameraCell = null;
+        LastCameraCellResolution = CameraCellResolution.None;
         return null;
     }
 
diff --git a/src/AcDream.App/Rendering/GameWindow.cs b/src/AcDream.App/Rendering/GameWindow.cs
index cf84efe..8cc0ce6 100644
--- a/src/AcDream.App/Rendering/GameWindow.cs
+++ b/src/AcDream.App/Rendering/GameWindow.cs
@@ -7315,6 +7315,22 @@ public sealed class GameWindow : IDisposable
                         clipAssembly.OutsidePlaneCount,
                         clipAssembly.PerCellPlaneCounts,
                         clipAssembly.ScissorFallbacks);
+
+                // Phase U.4c flap probe (ACDREAM_PROBE_FLAP) — paired with the builder's
+                // per-frame [flap] line. res = which FindCameraCell branch chose the root;
+                // eyeInRoot = is the EYE actually inside clipRoot's AABB (n ⇒ stale root via
+                // cache/grace, the leading flap hypothesis); terrain/outVisible = the frame's
+                // outcome (Skip/false ⇒ terrain+shells flapped off this frame).
+                if (AcDream.Core.Rendering.RenderingDiagnostics.ProbeFlapEnabled)
+                {
+                    var flapPlayer = _playerController?.Position ?? camPos;
+                    bool eyeInRoot = CellVisibility.PointInCell(camPos, clipRoot);
+                    Console.WriteLine(
+                        $"[flap-cam] root=0x{clipRoot.CellId:X8} res={_cellVisibility.LastCameraCellResolution} " +
+                        $"eyeInRoot={(eyeInRoot ? "Y" : "n")} eye=({camPos.X:F2},{camPos.Y:F2},{camPos.Z:F2}) " +
+                        $"player=({flapPlayer.X:F2},{flapPlayer.Y:F2},{flapPlayer.Z:F2}) " +
+                        $"terrain={clipAssembly.TerrainMode} outVisible={clipAssembly.OutdoorVisible}");
+                }
             }
             else
             {
diff --git a/src/AcDream.App/Rendering/PortalVisibilityBuilder.cs b/src/AcDream.App/Rendering/PortalVisibilityBuilder.cs
index 5276196..e7f47d1 100644
--- a/src/AcDream.App/Rendering/PortalVisibilityBuilder.cs
+++ b/src/AcDream.App/Rendering/PortalVisibilityBuilder.cs
@@ -230,9 +230,59 @@ public static class PortalVisibilityBuilder
         if (pvDump)
             Console.WriteLine($"[pv-dump] OUTSIDEVIEW polys={frame.OutsideView.Polygons.Count} bfsCellViews={frame.CellViews.Count} crossBldg={frame.CrossBuildingViews.Count}");
 
+        // Phase U.4c flap probe (ACDREAM_PROBE_FLAP) — read-only per-frame snapshot of the
+        // root cell's per-portal side-test + projection + the frame's exit/visible counts.
+        if (AcDream.Core.Rendering.RenderingDiagnostics.ProbeFlapEnabled)
+            EmitFlapProbe(cameraCell, cameraPos, viewProj, frame);
+
         return frame;
     }
 
+    // Phase U.4c flap probe. One [flap] line per Build: the root cell's per-portal
+    // signed distance D (eye→portal plane), traverse/cull decision, and NDC projection
+    // vertex count, plus the frame's OutsideView polygon count + visible-cell count.
+    // `localEye` is the eye in root-local space — its component along an interior portal
+    // plane reveals when the eye has crossed past that plane (the stale-root region that
+    // makes the side test cull a still-needed portal). Read-only recompute; no effect on
+    // the returned frame. Throwaway apparatus — strip with the probe.
+    private static void EmitFlapProbe(
+        LoadedCell cameraCell, Vector3 cameraPos, Matrix4x4 viewProj, PortalVisibilityFrame frame)
+    {
+        var localEye = Vector3.Transform(cameraPos, cameraCell.InverseWorldTransform);
+        var sb = new System.Text.StringBuilder(220);
+        sb.Append("[flap] root=0x").Append(cameraCell.CellId.ToString("X8"));
+        sb.Append(" eye=(").Append(cameraPos.X.ToString("F2")).Append(',')
+          .Append(cameraPos.Y.ToString("F2")).Append(',').Append(cameraPos.Z.ToString("F2")).Append(')');
+        sb.Append(" localEye=(").Append(localEye.X.ToString("F2")).Append(',')
+          .Append(localEye.Y.ToString("F2")).Append(',').Append(localEye.Z.ToString("F2")).Append(')');
+        for (int i = 0; i < cameraCell.Portals.Count; i++)
+        {
+            var portal = cameraCell.Portals[i];
+            float d = float.NaN;
+            bool side = true;
+            if (i < cameraCell.ClipPlanes.Count && cameraCell.ClipPlanes[i].Normal.LengthSquared() >= 1e-8f)
+            {
+                var pl = cameraCell.ClipPlanes[i];
+                d = Vector3.Dot(pl.Normal, localEye) + pl.D;
+                side = CameraOnInteriorSide(cameraCell, i, cameraPos);
+            }
+            int projN = -1;
+            if (i < cameraCell.PortalPolygons.Count)
+            {
+                var poly = cameraCell.PortalPolygons[i];
+                if (poly != null && poly.Length >= 3)
+                    projN = PortalProjection.ProjectToNdc(poly, cameraCell.WorldTransform, viewProj).Length;
+            }
+            sb.Append(" | p").Append(i).Append("->0x").Append(portal.OtherCellId.ToString("X4"));
+            sb.Append(" D=").Append(float.IsNaN(d) ? "na" : d.ToString("F2"));
+            sb.Append(side ? " TRV" : " CULL");
+            sb.Append(" proj=").Append(projN);
+        }
+        sb.Append(" || outPolys=").Append(frame.OutsideView.Polygons.Count);
+        sb.Append(" vis=").Append(frame.OrderedVisibleCells.Count);
+        Console.WriteLine(sb.ToString());
+    }
+
     // Mirrors CellVisibility's portal-side test (InsideSide convention).
     private static bool CameraOnInteriorSide(LoadedCell cell, int portalIndex, Vector3 cameraPos)
     {
diff --git a/src/AcDream.Core/Rendering/RenderingDiagnostics.cs b/src/AcDream.Core/Rendering/RenderingDiagnostics.cs
index b373aac..14687b9 100644
--- a/src/AcDream.Core/Rendering/RenderingDiagnostics.cs
+++ b/src/AcDream.Core/Rendering/RenderingDiagnostics.cs
@@ -96,6 +96,24 @@ public static class RenderingDiagnostics
     public static bool ProbeVisibilityEnabled { get; set; } =
         Environment.GetEnvironmentVariable("ACDREAM_PROBE_VIS") == "1";
 
+    /// <summary>
+    /// Phase U.4c (2026-05-31) flap-convergence probe. When true, the portal
+    /// visibility pass emits, EVERY frame the camera root is an indoor cell, a
+    /// <c>[flap]</c> line (root cell's per-portal side-test D + traverse/cull +
+    /// projection, plus the frame's OutsideView/visible counts) and the call site
+    /// emits a paired <c>[flap-cam]</c> line (FindCameraCell resolution reason,
+    /// camera EYE worldpos, player worldpos, eye-in-root-AABB flag). Unlike the
+    /// cell-change-throttled <see cref="ProbeVisibilityEnabled"/> probe, this fires
+    /// per-frame so it captures the flicker (the exit cell dropping in/out at a
+    /// STABLE root). Pinpoints WHY the exit cell drops: side-test cull (eye past an
+    /// interior portal plane), empty projection, or a stale root (eye outside the
+    /// cell while FindCameraCell still reports it via cache/grace). Throwaway
+    /// apparatus — strip once the flap mechanism is confirmed.
+    /// Initial state from <c>ACDREAM_PROBE_FLAP=1</c>.
+    /// </summary>
+    public static bool ProbeFlapEnabled { get; set; } =
+        Environment.GetEnvironmentVariable("ACDREAM_PROBE_FLAP") == "1";
+
     // Cell-change gate for EmitVis. The probe fires once per distinct root cell
     // so launch.log stays readable under motion (the per-frame call is a no-op
     // when the root is unchanged). Sentinel 0 = "no root yet" — the first real