Expose `previousSibling` and `nextSibling` to ActionScript.

This commit also fixes a bug caused by excessive use of copypaste, which was detected by the included test.

core/src/avm1/globals/xml.rs

@@ -299,6 +299,48 @@ pub fn create_xmlnode_proto<'gc>(
         None,
         ReadOnly.into(),
     );
+    xmlnode_proto.add_property(
+        gc_context,
+        "previousSibling",
+        Executable::Native(|avm, ac, this: Object<'gc>, _args| {
+            if let Some(node) = this.as_xml_node() {
+                return Ok(node
+                    .prev_sibling()
+                    .unwrap_or(None)
+                    .map(|mut parent| {
+                        parent
+                            .script_object(ac.gc_context, Some(avm.prototypes.xml_node))
+                            .into()
+                    })
+                    .unwrap_or_else(|| Value::Null.into()));
+            }
+
+            Ok(Value::Undefined.into())
+        }),
+        None,
+        ReadOnly.into(),
+    );
+    xmlnode_proto.add_property(
+        gc_context,
+        "nextSibling",
+        Executable::Native(|avm, ac, this: Object<'gc>, _args| {
+            if let Some(node) = this.as_xml_node() {
+                return Ok(node
+                    .next_sibling()
+                    .unwrap_or(None)
+                    .map(|mut parent| {
+                        parent
+                            .script_object(ac.gc_context, Some(avm.prototypes.xml_node))
+                            .into()
+                    })
+                    .unwrap_or_else(|| Value::Null.into()));
+            }
+
+            Ok(Value::Undefined.into())
+        }),
+        None,
+        ReadOnly.into(),
+    );
     xmlnode_proto
         .as_script_object()
         .unwrap()

core/src/xml/tree.rs

@@ -320,7 +320,7 @@ impl<'gc> XMLNode<'gc> {
                     .checked_sub(1)
                     .and_then(|p| children.get(p).cloned());
                 let new_next = new_child_position
-                    .checked_sub(1)
+                    .checked_add(1)
                     .and_then(|p| children.get(p).cloned());
 
                 child.adopt_siblings(mc, new_prev, new_next)?

core/tests/regression_tests.rs

@@ -111,6 +111,7 @@ swf_tests! {
     (xml_has_child_nodes, "avm1/xml_has_child_nodes", 1),
     (xml_first_last_child, "avm1/xml_first_last_child", 1),
     (xml_parent_and_child, "avm1/xml_parent_and_child", 1),
+    (xml_siblings, "avm1/xml_siblings", 1),
 }
 
 #[test]

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

@@ -0,0 +1,10 @@
+null
+true
+true
+true
+true
+true
+true
+null
+null
+null