diff --git a/tests/AcDream.Core.Tests/Rendering/CellGraphRootTests.cs b/tests/AcDream.Core.Tests/Rendering/CellGraphRootTests.cs
new file mode 100644
index 0000000..2b75717
--- /dev/null
+++ b/tests/AcDream.Core.Tests/Rendering/CellGraphRootTests.cs
@@ -0,0 +1,208 @@
+// CellGraphRootTests.cs — Stage 3 (2026-06-02): unit tests for render-root
+// selection logic and CellGraph.FindVisibleChildCell.
+//
+// Tests 1–3: render-branch predicates. These exercise the formulas in
+// GameWindow.OnRender directly (Stage 3):
+//   rootSeenOutside = physicsRoot?.SeenOutside ?? true
+//   playerInsideCell = cameraInsideCell && !rootSeenOutside
+//   renderSky = !cameraInsideCell || rootSeenOutside
+//
+// Retail anchors:
+//   CellManager::ChangePosition @ 0x004559B0 (pseudo_c:94649) — landscape kept
+//     live iff seen_outside; sun zeroed when inside a sealed interior.
+//   SmartBox::RenderNormalMode @ 0x00453aa0 (pseudo_c:92635) — sky gate.
+//
+// Tests 4–6: CellGraph.FindVisibleChildCell (retail find_visible_child_cell
+// @ 0x0052dc50, pseudo_c:311397).
+
+using System.Collections.Generic;
+using System.Numerics;
+using AcDream.Core.World.Cells;
+using Xunit;
+
+namespace AcDream.Core.Tests.Rendering;
+
+public class CellGraphRootTests
+{
+    // ------------------------------------------------------------------
+    // Helpers
+    // ------------------------------------------------------------------
+
+    /// <summary>
+    /// Synthetic EnvCell with an identity transform and axis-aligned bounds so
+    /// PointInCell returns true for points inside [min, max].
+    /// seenOutside = false → sealed dungeon; true → building interior/exterior.
+    /// </summary>
+    private static EnvCell MakeEnvCell(uint id, Vector3 min, Vector3 max, bool seenOutside = false)
+        => new EnvCell(
+            id,
+            Matrix4x4.Identity,
+            Matrix4x4.Identity,
+            min, max,
+            portals: new List<CellPortal>(),
+            stabList: new List<uint>(),
+            seenOutside: seenOutside,
+            containmentBsp: null);
+
+    /// <summary>
+    /// EnvCell with an explicit stab list (used by FindVisibleChildCell tests).
+    /// </summary>
+    private static EnvCell MakeEnvCellWithStab(uint id, Vector3 min, Vector3 max,
+        IReadOnlyList<uint> stabList, bool seenOutside = false)
+        => new EnvCell(
+            id,
+            Matrix4x4.Identity,
+            Matrix4x4.Identity,
+            min, max,
+            portals: new List<CellPortal>(),
+            stabList: stabList,
+            seenOutside: seenOutside,
+            containmentBsp: null);
+
+    // ------------------------------------------------------------------
+    // Predicate helpers — mirror the formulas in GameWindow.OnRender (Stage 3)
+    // ------------------------------------------------------------------
+
+    private static bool RootSeenOutside(EnvCell? physicsRoot) => physicsRoot?.SeenOutside ?? true;
+
+    // Retail CellManager::ChangePosition (0x004559B0): sun/sun-ambient zeroed
+    // when inside a sealed interior (cameraInsideCell=true, seen_outside=false).
+    // Building interiors with seen_outside keep the sun; outdoor root always keeps it.
+    private static bool PlayerInsideCell(bool cameraInsideCell, bool rootSeenOutside)
+        => cameraInsideCell && !rootSeenOutside;
+
+    // Stage 3 sky gate: visible unless inside a sealed dungeon.
+    private static bool RenderSky(bool cameraInsideCell, bool rootSeenOutside)
+        => !cameraInsideCell || rootSeenOutside;
+
+    // ------------------------------------------------------------------
+    // Test 1: outdoor root (null CurrCell) — seen_outside=true, sky rendered,
+    //         player NOT considered inside a cell.
+    // ------------------------------------------------------------------
+
+    [Fact]
+    public void RootSelection_OutdoorRoot_NullCurrCell_SeenOutsideDefaultsToTrue()
+    {
+        // When physicsRoot==null (pre-spawn, or outdoor landcell) the ?? true
+        // gives rootSeenOutside=true; cameraInsideCell=false (no cell).
+        bool rootSeenOutside = RootSeenOutside(null);
+        bool cameraInsideCell = false;   // no indoor root → ComputeVisibilityFromRoot returns null
+
+        bool playerInsideCell = PlayerInsideCell(cameraInsideCell, rootSeenOutside);
+        bool renderSky        = RenderSky(cameraInsideCell, rootSeenOutside);
+
+        Assert.True(rootSeenOutside,    "outdoor null root → rootSeenOutside=true");
+        Assert.False(playerInsideCell,  "outdoor → playerInsideCell=false (sun kept live)");
+        Assert.True(renderSky,          "outdoor → sky rendered");
+    }
+
+    // ------------------------------------------------------------------
+    // Test 2: building interior (seen_outside=true) — sky rendered (interim
+    //         full-screen until Stage 4 clips it), sun kept live.
+    // ------------------------------------------------------------------
+
+    [Fact]
+    public void RootSelection_BuildingInterior_SeenOutside_SkyRenderedAndSunKept()
+    {
+        // A cottage cell: seen_outside=true (exit portal exists).
+        var physicsRoot = MakeEnvCell(0xA9B40170u, Vector3.Zero, new Vector3(10, 10, 10),
+            seenOutside: true);
+
+        bool rootSeenOutside  = RootSeenOutside(physicsRoot);
+        bool cameraInsideCell = true;   // inside a cell → ComputeVisibilityFromRoot non-null
+
+        bool playerInsideCell = PlayerInsideCell(cameraInsideCell, rootSeenOutside);
+        bool renderSky        = RenderSky(cameraInsideCell, rootSeenOutside);
+
+        Assert.True(rootSeenOutside,    "building interior seen_outside=true");
+        Assert.False(playerInsideCell,  "seen_outside=true → sun kept live");
+        Assert.True(renderSky,          "seen_outside=true → sky rendered (Stage 4 will clip to doorway)");
+    }
+
+    // ------------------------------------------------------------------
+    // Test 3: sealed dungeon (seen_outside=false) — sky suppressed, sun zeroed.
+    // ------------------------------------------------------------------
+
+    [Fact]
+    public void RootSelection_Dungeon_NoSeenOutside_SkyNotRenderedAndSunZeroed()
+    {
+        // A dungeon cell: seen_outside=false (no exit portal reachable).
+        var physicsRoot = MakeEnvCell(0x01D90100u, Vector3.Zero, new Vector3(10, 10, 10),
+            seenOutside: false);
+
+        bool rootSeenOutside  = RootSeenOutside(physicsRoot);
+        bool cameraInsideCell = true;   // inside a cell → non-null visibility result
+
+        bool playerInsideCell = PlayerInsideCell(cameraInsideCell, rootSeenOutside);
+        bool renderSky        = RenderSky(cameraInsideCell, rootSeenOutside);
+
+        Assert.False(rootSeenOutside,   "dungeon seen_outside=false");
+        Assert.True(playerInsideCell,   "sealed dungeon → playerInsideCell=true (sun zeroed)");
+        Assert.False(renderSky,         "sealed dungeon → sky suppressed");
+    }
+
+    // ------------------------------------------------------------------
+    // Test 4: FindVisibleChildCell — query inside the root cell → returns root.
+    // ------------------------------------------------------------------
+
+    [Fact]
+    public void FindVisibleChildCell_PlayerCellContains_ReturnsPlayerCell()
+    {
+        var graph = new CellGraph();
+        var root = MakeEnvCell(0xA9B40170u, Vector3.Zero, new Vector3(10, 10, 10));
+        graph.Add(root);
+
+        var point = new Vector3(5, 5, 5); // inside root's bounds
+        var result = graph.FindVisibleChildCell(root.Id, point);
+
+        Assert.NotNull(result);
+        Assert.Equal(root.Id, result!.Id);
+    }
+
+    // ------------------------------------------------------------------
+    // Test 5: FindVisibleChildCell — query inside a stab-list neighbour → returns that cell.
+    // ------------------------------------------------------------------
+
+    [Fact]
+    public void FindVisibleChildCell_StabListContains_ReturnsNeighbour()
+    {
+        var graph = new CellGraph();
+
+        // Root at [0,10]. Neighbour B at [20,30]. Root's stab list includes B.
+        uint rootId = 0xA9B40170u;
+        uint neighId = 0xA9B40171u;
+
+        var neigh = MakeEnvCell(neighId, new Vector3(20, 0, 0), new Vector3(30, 10, 10));
+        var root  = MakeEnvCellWithStab(rootId, Vector3.Zero, new Vector3(10, 10, 10),
+            stabList: new List<uint> { neighId });
+
+        graph.Add(root);
+        graph.Add(neigh);
+
+        var point = new Vector3(25, 5, 5); // inside neighbour, outside root
+        var result = graph.FindVisibleChildCell(rootId, point);
+
+        Assert.NotNull(result);
+        Assert.Equal(neighId, result!.Id);
+    }
+
+    // ------------------------------------------------------------------
+    // Test 6: FindVisibleChildCell — query outside all cells → null.
+    // ------------------------------------------------------------------
+
+    [Fact]
+    public void FindVisibleChildCell_NeitherContains_ReturnsNull()
+    {
+        var graph = new CellGraph();
+        uint rootId = 0xA9B40170u;
+
+        var root = MakeEnvCellWithStab(rootId, Vector3.Zero, new Vector3(10, 10, 10),
+            stabList: new List<uint>());
+        graph.Add(root);
+
+        var pointFarAway = new Vector3(999, 999, 999); // outside all cells
+        var result = graph.FindVisibleChildCell(rootId, pointFarAway);
+
+        Assert.Null(result);
+    }
+}