Viewpoint XML: write an enabled section box only as section_box

src/NwcIO/Model/Sectioning/NwcXmlViewpointSerializer.cs

@@ -99,17 +99,12 @@ private static XElement SerializeViewpoint(NwcSavedViewpoint vp)
             new XAttribute("guid", vp.Id.ToString()),
             camElem);
 
-        // Section planes
-        var allPlanes = new List<NwcSectionPlane>(vp.SectionPlanes);
-        if (vp.SectionBox is not null && vp.SectionBox.IsEnabled)
-        {
-            allPlanes.AddRange(vp.SectionBox.ToSectionPlanes());
-        }
-
-        if (allPlanes.Count > 0)
+        // Section planes: only the viewpoint's own. The section box is written once, as <section_box> below; its
+        // planes here would be read back as section planes, adding six more on every round trip.
+        if (vp.SectionPlanes.Count > 0)
         {
             var planesElem = new XElement("clip_planes");
-            foreach (var plane in allPlanes)
+            foreach (var plane in vp.SectionPlanes)
             {
                 planesElem.Add(new XElement("clip_plane",
                     new XAttribute("index", plane.PlaneIndex),

tests/NwcIO.Tests/XmlViewpointSerializerTests.cs

@@ -86,6 +86,38 @@ public void Serialize_Roundtrip_PreservesViewpointDetails()
         Assert.Equal(12.0, rArrow.EndPoint.X, 3);
     }
 
+    [Fact]
+    public void Serialize_SectionBoxAndPlane_RoundTripsUnchanged()
+    {
+        var bounds = new BoundingBox3d(new Vector3d(0, 0, 0), new Vector3d(1, 2, 3));
+        var top = NwcSectionPlane.CreateTop(4.5, 7);
+        var list = new List<NwcSavedViewpoint>
+        {
+            new()
+            {
+                Name = "v",
+                Camera = new NwcCamera(),
+                SectionPlanes = [top],
+                SectionBox = new NwcSectionBox(bounds, true),
+            },
+        };
+
+        for (int round = 0; round < 3; round++)
+        {
+            list = NwcXmlViewpointSerializer.Deserialize(NwcXmlViewpointSerializer.Serialize(list)).ToList();
+
+            var vp = Assert.Single(list);
+            var plane = Assert.Single(vp.SectionPlanes);
+            Assert.Equal(7, plane.PlaneIndex);
+            Assert.Equal(top.Normal, plane.Normal);
+            Assert.Equal(top.Distance, plane.Distance);
+            Assert.Equal(NwcSectionAlignment.Top, plane.Alignment);
+            Assert.NotNull(vp.SectionBox);
+            Assert.True(vp.SectionBox.IsEnabled);
+            Assert.Equal(bounds, vp.SectionBox.Bounds);
+        }
+    }
+
     [Fact]
     public void Serialize_EmptyList_ReturnsValidRootElement()
     {