appendChild also refuses to orphan nodes already part of another XML tree.

core/src/avm1/globals/xml.rs

@@ -53,8 +53,10 @@ pub fn xmlnode_append_child<'gc>(
         this.as_xml_node(),
         args.get(0).map(|n| n.as_object().map(|n| n.as_xml_node())),
     ) {
-        let position = xmlnode.children_len();
-        xmlnode.insert_child(ac.gc_context, position, child_xmlnode)?;
+        if let Ok(None) = child_xmlnode.parent() {
+            let position = xmlnode.children_len();
+            xmlnode.insert_child(ac.gc_context, position, child_xmlnode)?;
+        }
     }
 
     Ok(Value::Undefined.into())

core/tests/regression_tests.rs

@@ -114,6 +114,7 @@ swf_tests! {
     (xml_siblings, "avm1/xml_siblings", 1),
     (xml_attributes_read, "avm1/xml_attributes_read", 1),
     (xml_append_child, "avm1/xml_append_child", 1),
+    (xml_append_child_with_parent, "avm1/xml_append_child_with_parent", 1),
     (xml_remove_node, "avm1/xml_remove_node", 1),
     (xml_insert_before, "avm1/xml_insert_before", 1),
 }

core/tests/swfs/avm1/xml_append_child_with_parent/output.txt

@@ -0,0 +1,20 @@
+true
+null
+true
+true
+null
+true
+Swapping nodes...
+true
+null
+true
+true
+null
+true
+Failing to swap nodes...
+true
+null
+true
+true
+null
+true