fix: clean up orphaned BVH nodes when remove creates a partial root

src/partitioning/bvh/bvh_tests.rs

@@ -1,6 +1,8 @@
 use crate::bounding_volume::Aabb;
 use crate::math::{Real, Vector};
-use crate::partitioning::{Bvh, BvhBuildStrategy, BvhNode, BvhNodeIndex, TraversalAction};
+use crate::partitioning::{
+    Bvh, BvhBuildStrategy, BvhNode, BvhNodeIndex, BvhWorkspace, TraversalAction,
+};
 
 fn make_test_aabb(i: usize) -> Aabb {
     Aabb::from_half_extents(Vector::splat(i as Real).into(), Vector::splat(1.0))
@@ -242,3 +244,38 @@ fn bvh_build_and_removal() {
         }
     }
 }
+
+#[test]
+fn bvh_remove_to_partial_root_then_optimize() {
+    // Regression test for the bug reported in #409 where `Bvh::remove` would leave orphaned
+    // wide nodes in `self.nodes`/`self.parents` after reducing the tree to a
+    // partial root (a single surviving leaf at node 0). Earlier removals on a
+    // tree with more than one wide node would intentionally leave orphans for
+    // the next `refit` to compact, but if a partial root was created before
+    // any refit, those orphans remained reachable from `self.nodes` and
+    // `optimize_incremental` would walk them as if they were live, crashing
+    // on the corrupt structure.
+    //
+    // We pick enough leaves to ensure at least one orphan-leaving removal
+    // (`wide_node_index != 0`) before the final partial-root removal.
+    let leaves: std::vec::Vec<_> = (0..10).map(make_test_aabb).collect();
+    let mut bvh = Bvh::from_leaves(BvhBuildStrategy::Binned, &leaves);
+
+    // Remove all but the last leaf, without ever calling refit in between.
+    for i in 0..(leaves.len() as u32 - 1) {
+        bvh.remove(i);
+    }
+
+    // After the final remove, the tree should be a partial root with exactly
+    // one surviving leaf, and no orphaned wide nodes left over.
+    assert_eq!(bvh.leaf_count(), 1);
+
+    // Without the fix this call walks the orphan-laden tree as if it were
+    // live and ends up corrupting/crashing on the partial root.
+    let mut workspace = BvhWorkspace::default();
+    bvh.optimize_incremental(&mut workspace);
+
+    assert_eq!(bvh.nodes.len(), 1);
+    assert_eq!(bvh.parents.len(), 1);
+    bvh.assert_well_formed();
+}

src/partitioning/bvh/bvh_tree.rs

@@ -2383,6 +2383,14 @@ impl Bvh {
 
                     // Now we can just clear the right leaf.
                     self.nodes[0].right = BvhNode::zeros();
+
+                    // Clean up orphaned nodes. With a partial root, only node[0] is
+                    // reachable. Previous removes may have left orphaned wide nodes
+                    // that were waiting for refit to compact them. If we don't truncate
+                    // here, the tree appears as a single-leaf tree with unreachable
+                    // nodes, which corrupts optimize_incremental.
+                    self.nodes.truncate(1);
+                    self.parents.truncate(1);
                 } else {
                     // The sibling isn’t a leaf. It becomes the new root at index 0.
                     self.nodes[0] = self.nodes[self.nodes[sibling].children as usize];