fix #451: stabilize portal seam rendering

tests/AcDream.App.Tests/Rendering/ClipFrameAssemblerTests.cs

@@ -128,6 +128,39 @@ public class ClipFrameAssemblerTests
         Assert.Equal(TerrainClipMode.Planes, asm.TerrainMode);
     }
 
+    [Fact]
+    public void AppendLookInFrames_PacksEveryNestedCellViewWithoutReplacingMainRoutes()
+    {
+        const uint mainCell = 0xA9B40100;
+        const uint lookInCell = 0xA9B40106;
+        var main = new PortalVisibilityFrame();
+        main.CellViews[mainCell] = ViewOf(Square(0f, 0f, 0.8f));
+        main.OrderedVisibleCells.Add(mainCell);
+        main.OutsideView.Add(Square(0f, 0f, 0.7f));
+
+        var nested = new PortalVisibilityFrame();
+        nested.CellViews[lookInCell] = ViewOf(
+            Square(-0.35f, 0f, 0.15f),
+            Square(0.35f, 0f, 0.15f));
+        nested.OrderedVisibleCells.Add(lookInCell);
+
+        using ClipFrame frame = ClipFrame.NoClip();
+        ClipFrameAssembly assembly = ClipFrameAssembler.Assemble(frame, main);
+        int mainSlot = assembly.CellIdToSlot[mainCell];
+        int slotsBeforeLookIn = frame.SlotCount;
+
+        ClipFrameAssembler.AppendLookInFrames(frame, [nested], assembly);
+
+        var key = new LookInClipCell(0, lookInCell);
+        ClipViewSlice[] slices = assembly.LookInCellToViewSlices[key];
+        Assert.Equal(2, slices.Length);
+        Assert.All(slices, slice => Assert.True(slice.Slot >= slotsBeforeLookIn));
+        Assert.NotEqual(slices[0].Slot, slices[1].Slot);
+        Assert.Equal(slices[0].Slot, assembly.LookInCellToSlot[key]);
+        Assert.Equal(mainSlot, assembly.CellIdToSlot[mainCell]);
+        Assert.DoesNotContain(lookInCell, assembly.CellIdToSlot.Keys);
+    }
+
     [Fact]
     public void Assemble_OutsideViewWithExitPortal_HasOutsideViewTrue_AabbMatchesBounds()
     {