diff --git a/ipywidgets b/ipywidgets
new file mode 160000
index 0000000000..511663a563
--- /dev/null
+++ b/ipywidgets
@@ -0,0 +1 @@
+Subproject commit 511663a56324cea5324f49a65ebe25e2f1b04d87
diff --git a/packages/base/src/widget.ts b/packages/base/src/widget.ts
index cfea6ef8fe..743b0365a8 100644
--- a/packages/base/src/widget.ts
+++ b/packages/base/src/widget.ts
@@ -34,12 +34,6 @@ import { KernelMessage } from '@jupyterlab/services';
  */
 const IPY_MODEL_ = 'IPY_MODEL_';
 
-/**
- * A best-effort method for performing deep copies.
- */
-const deepcopyJSON = (x: JSONValue) => JSON.parse(JSON.stringify(x));
-
-const deepcopy = globalThis.structuredClone || deepcopyJSON;
 
 /**
  * Replace model ids with models recursively.
@@ -577,13 +571,14 @@ export class WidgetModel extends Backbone.Model {
 
         if (serialize == null && keySerializers.deserialize === unpack_models) {
           // handle https://github.com/jupyter-widgets/ipywidgets/issues/3735
-          serialize = deepcopyJSON;
+          serialize = JSONExt.deepCopy;
         }
 
         if (serialize) {
           state[k] = serialize(state[k], this);
         } else {
           // the default serializer just deep-copies the object
+          const deepcopy = globalThis.structuredClone || JSONExt.deepCopy;
           state[k] = deepcopy(state[k]);
         }