fix(custom-elements): preserve prototypes on failures before super

Only finalize failed upgrade wrappers if the construction stack shows that HTMLElement construction consumed the pending wrapper. Throwing or returning a different object before super must leave the original prototype intact.

Add main-window and iframe regressions for both failure paths, including discarded attribute and connected reactions.

moli-renderer-v8/src/custom_elements/existing_upgrade_invocation.rs

@@ -31,7 +31,11 @@ pub(super) fn upgrade_existing_custom_element_with_constructor<'s>(
         .custom_elements_mut_for_node_handle(handle)
         .begin_construction(scope, constructor, wrapper, handle);
     let _dynamic_markup = enter_upgrade_dynamic_markup_insertion(host_ptr, handle);
-    match invoke_custom_element_constructor(scope, host_ptr, constructor) {
+    let invocation = invoke_custom_element_constructor(scope, host_ptr, constructor);
+    let consumed_pending_wrapper = unsafe { &*host_ptr }
+        .custom_elements_for_node_handle(handle)
+        .is_some_and(|store| store.pending_construction_is_already_constructed(handle));
+    match invocation {
         CustomElementConstructorInvocation::Created(created) => {
             let created = v8::Local::new(scope, &created);
             if !validate_existing_custom_element_upgrade_result(wrapper, created) {
@@ -45,7 +49,9 @@ pub(super) fn upgrade_existing_custom_element_with_constructor<'s>(
                     ),
                     failure_prototype,
                 );
-                finalize_wrapper_custom_element_prototype(scope, wrapper, constructor);
+                if consumed_pending_wrapper {
+                    finalize_wrapper_custom_element_prototype(scope, wrapper, constructor);
+                }
                 return true;
             }
             finalize_wrapper_custom_element_prototype(scope, wrapper, constructor);
@@ -53,10 +59,13 @@ pub(super) fn upgrade_existing_custom_element_with_constructor<'s>(
             true
         }
         CustomElementConstructorInvocation::Exception(exception) => {
-            finalize_wrapper_custom_element_prototype(scope, wrapper, constructor);
-            if let Some(canonical) = unsafe { &mut *host_ptr }
-                .native_bridge_mut()
-                .wrap_handle(scope, host_ptr, handle)
+            if consumed_pending_wrapper {
+                finalize_wrapper_custom_element_prototype(scope, wrapper, constructor);
+            }
+            if consumed_pending_wrapper
+                && let Some(canonical) = unsafe { &mut *host_ptr }
+                    .native_bridge_mut()
+                    .wrap_handle(scope, host_ptr, handle)
             {
                 if canonical.strict_equals(wrapper.into()) {
                     synchronize_custom_element_prototype_between_wrappers(
@@ -90,7 +99,9 @@ pub(super) fn upgrade_existing_custom_element_with_constructor<'s>(
                 ConstructionFailure::Exception(exception),
                 failure_prototype,
             );
-            finalize_wrapper_custom_element_prototype(scope, wrapper, constructor);
+            if consumed_pending_wrapper {
+                finalize_wrapper_custom_element_prototype(scope, wrapper, constructor);
+            }
             true
         }
         CustomElementConstructorInvocation::Empty => {

moli-renderer-v8/src/script_vm/tests/dom_elements/custom_elements.rs

@@ -6889,6 +6889,58 @@ fn custom_element_constructor_error_reports_to_definition_window() {
     );
 }
 
+#[test]
+fn failed_existing_upgrade_before_super_keeps_original_prototype() {
+    let mut vm = new_storage_test_vm("https://ce-before-super.test/");
+    let result = vm.eval(r#"
+        (() => {
+          window.addEventListener("error", event => event.preventDefault());
+          const html = document.documentElement || document.appendChild(document.createElement("html"));
+          const body = document.body || html.appendChild(document.createElement("body"));
+          const frame = body.appendChild(document.createElement("iframe"));
+          const results = [];
+          for (const [realm, w] of [["main", window], ["child", frame.contentWindow]]) {
+            for (const failure of ["throw", "return-object"]) {
+              const doc = w.document;
+              const name = `before-super-${failure}`;
+              const element = doc.createElement(name);
+              element.setAttribute("data-value", "before");
+              (doc.body || doc.documentElement || doc).appendChild(element);
+              const original = Object.getPrototypeOf(element);
+              const log = [];
+              class FailedBeforeSuper extends w.HTMLElement {
+                constructor() {
+                  log.push("constructor");
+                  if (failure === "throw") throw new Error("before super");
+                  return {};
+                }
+                static get observedAttributes() { return ["data-value"]; }
+                attributeChangedCallback() { log.push("attribute"); }
+                connectedCallback() { log.push("connected"); }
+              }
+              w.customElements.define(name, FailedBeforeSuper);
+              results.push({realm, failure,
+                unchanged: Object.getPrototypeOf(element) === original,
+                custom: element instanceof FailedBeforeSuper, log});
+            }
+          }
+          return JSON.stringify(results);
+        })()
+    "#).expect("failed upgrades before super should report their original prototypes");
+    let results: serde_json::Value = serde_json::from_str(&result).expect("upgrade results");
+    let results = results.as_array().expect("four upgrade cases");
+    assert_eq!(results.len(), 4);
+    for result in results {
+        assert_eq!(result["unchanged"], true, "{result}");
+        assert_eq!(result["custom"], false, "{result}");
+        assert_eq!(
+            result["log"],
+            serde_json::json!(["constructor"]),
+            "{result}"
+        );
+    }
+}
+
 #[test]
 fn failed_existing_upgrade_preserves_definition_prototype_and_clears_reactions() {
     let mut vm = new_storage_test_vm("https://ce-failed-upgrade.test/");