fix(scene-graph): validate reorder destinations

packages/scene-graph/src/index.ts

@@ -523,16 +523,16 @@ export class SceneGraph {
 
   insertChildAt(childId: string, parentId: string, index: number): void {
     const node = this.getNode(childId)
-    const previousParentId = node?.parentId ?? null
+    const newParent = this.getNode(parentId)
+    if (!node || !newParent) return
+    const previousParentId = node.parentId
     const oldParent = previousParentId ? this.getNode(previousParentId) : undefined
     if (oldParent) {
       oldParent.childIds = oldParent.childIds.filter((id) => id !== childId)
     }
-    const newParent = this.getNode(parentId)
-    if (!newParent) return
     newParent.childIds = newParent.childIds.filter((id) => id !== childId)
     newParent.childIds.splice(index, 0, childId)
-    if (node) node.parentId = parentId
+    node.parentId = parentId
     this.clearAbsPosCache()
     this.emitter.emit('node:reordered', childId, parentId, index, previousParentId)
   }

packages/scene-graph/src/mutation-impact.test.ts

@@ -54,6 +54,29 @@ describe('scene mutation impact', () => {
     expect(impact.currentParentIds).toContain(right.id)
   })
 
+  test('retains old and new parents when reorderChild transfers a node', async () => {
+    const { graph, left, right, child } = transferSetup()
+
+    const { impact } = await collectSceneMutation(graph, () =>
+      graph.reorderChild(child.id, right.id, 0)
+    )
+
+    expect(child.parentId).toBe(right.id)
+    expect(left.childIds).not.toContain(child.id)
+    expect(right.childIds).toContain(child.id)
+    expect(impact.previousParentIds).toContain(left.id)
+    expect(impact.currentParentIds).toContain(right.id)
+  })
+
+  test('does not detach insertChildAt targets when the destination is missing', () => {
+    const { graph, left, child } = transferSetup()
+
+    graph.insertChildAt(child.id, 'missing', 0)
+
+    expect(child.parentId).toBe(left.id)
+    expect(left.childIds).toContain(child.id)
+  })
+
   test('retains old and new parents when insertChildAt transfers a node', async () => {
     const { graph, left, right, child } = transferSetup()