diff --git a/tests/AcDream.App.Tests/Rendering/UnifiedFloodTests.cs b/tests/AcDream.App.Tests/Rendering/UnifiedFloodTests.cs
new file mode 100644
index 00000000..75323d17
--- /dev/null
+++ b/tests/AcDream.App.Tests/Rendering/UnifiedFloodTests.cs
@@ -0,0 +1,63 @@
+using System;
+using System.Numerics;
+using AcDream.App.Rendering;
+using Xunit;
+
+namespace AcDream.App.Tests.Rendering;
+
+public class UnifiedFloodTests
+{
+    // Shared building fixture: a building cell whose interior is at Y >= 5.
+    // The exit portal faces -Y (Normal=(0,-1,0)); interior is where dot<=0 -> Y>=5 (InsideSide=1).
+    // The outdoor camera is at Y=-3, which is the OUTDOOR side (Y<5).
+    // OutdoorCellNode.Build flips InsideSide to 0 so the outdoor camera (dot>=0 i.e. Y<5) passes
+    // the side test and the flood reaches the building.
+    private static LoadedCell MakeBuildingCell(uint cellId)
+    {
+        var building = new LoadedCell { CellId = cellId, SeenOutside = true };
+        building.WorldTransform = Matrix4x4.Identity;
+        building.InverseWorldTransform = Matrix4x4.Identity;
+        building.Portals.Add(new CellPortalInfo(0xFFFF, 0, 0, 0));
+        // InsideSide=1: interior where (0,-1,0)·p+5 <= 0 -> Y>=5 (the building body is at Y>=5).
+        building.ClipPlanes.Add(new PortalClipPlane { Normal = new Vector3(0, -1, 0), D = 5f, InsideSide = 1 });
+        building.PortalPolygons.Add(new[]
+        {
+            new Vector3(-1, 5, 0), new Vector3(1, 5, 0), new Vector3(1, 5, 2), new Vector3(-1, 5, 2)
+        });
+        return building;
+    }
+
+    [Fact]
+    public void Build_RootedAtOutdoorNode_FloodsIntoBuilding()
+    {
+        var building = MakeBuildingCell(0xA9B40170);
+        var node = OutdoorCellNode.Build(0xA9B40031, new[] { building });
+        LoadedCell? Lookup(uint id) => (id & 0xFFFFu) == 0x0170 ? building : null;
+
+        var eye = new Vector3(0, -3, 1);
+        var view = Matrix4x4.CreateLookAt(eye, new Vector3(0, 5, 1), Vector3.UnitZ);
+        var proj = Matrix4x4.CreatePerspectiveFieldOfView(MathF.PI / 3f, 16f / 9f, 1f, 5000f);
+
+        var frame = PortalVisibilityBuilder.Build(node, eye, Lookup, view * proj);
+
+        Assert.Contains(0xA9B40031u, frame.OrderedVisibleCells); // the outdoor node itself
+        Assert.Contains(0xA9B40170u, frame.OrderedVisibleCells); // flooded into the building
+    }
+
+    [Fact]
+    public void Build_OutdoorBuildingCycle_Terminates()
+    {
+        // Building's exit portal reciprocally points back near the node; assert Build
+        // returns (does not hang) and the visible set is bounded/small.
+        var building = MakeBuildingCell(0xA9B40170);
+        var node = OutdoorCellNode.Build(0xA9B40031, new[] { building });
+        LoadedCell? Lookup(uint id) => (id & 0xFFFFu) == 0x0170 ? building
+                                     : (id & 0xFFFFu) == 0x0031 ? node : null;
+        var eye = new Vector3(0, -3, 1);
+        var view = Matrix4x4.CreateLookAt(eye, new Vector3(0, 5, 1), Vector3.UnitZ);
+        var proj = Matrix4x4.CreatePerspectiveFieldOfView(MathF.PI / 3f, 16f / 9f, 1f, 5000f);
+
+        var frame = PortalVisibilityBuilder.Build(node, eye, Lookup, view * proj);
+        Assert.True(frame.OrderedVisibleCells.Count < 10); // bounded, no runaway
+    }
+}