[scoped-custom-element-registry] Safari Tech Preview 214 fix (#614)

Defines rather than assigns shadowRoot.customElements

Author: sorvell

packages/scoped-custom-element-registry/CHANGELOG.md

@@ -10,6 +10,12 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 <!-- ### Changed -->
 <!-- ### Fixed -->
 
+## Unreleased
+
+### Fixed
+
+- Fixes [issue](https://github.com/webcomponents/polyfills/issues/613) with setting `shadowRoot.customElements` on Safari's native implementation
+
 ## [0.0.10] - 2025-02-26
 
 ### Added

packages/scoped-custom-element-registry/src/scoped-custom-element-registry.ts

@@ -681,7 +681,13 @@ Element.prototype.attachShadow = function (
     ...(args as [])
   ) as ShadowRootWithSettableCustomElements;
   if (registry !== undefined) {
-    shadowRoot['customElements'] = shadowRoot['registry'] = registry;
+    const descriptor = {
+      value: registry,
+      configurable: true,
+      writable: true,
+    };
+    Object.defineProperty(shadowRoot, 'customElements', descriptor);
+    Object.defineProperty(shadowRoot, 'registry', descriptor);
   }
   return shadowRoot;
 };