LibJS: Also apply AD-HOC execution context fixup on host defined path

As described in the comments, this was a bug in the shadow realm spec.
For now, just apply a fixup to the execution context so that it works as
intended.

Once tc39/proposal-shadowrealm#392 is updated to
match these changes to the spec we should be able to implement this in a
better way :^)

Userland/Libraries/LibJS/Runtime/ShadowRealmConstructor.cpp

@@ -39,6 +39,14 @@ ThrowCompletionOr<Value> ShadowRealmConstructor::call()
     return vm.throw_completion<TypeError>(ErrorType::ConstructorWithoutNew, vm.names.ShadowRealm);
 }
 
+// https://github.com/tc39/proposal-shadowrealm/pull/392
+// The default implementation of HostInitializeShadowRealm performs the following steps when called:
+static NonnullGCPtr<Object> default_create_global_object(JS::Realm& realm)
+{
+    // Return NormalCompletion(OrdinaryObjectCreate(realm.[[Intrinsics]].[[%Object.prototype%]])).
+    return Object::create(realm, realm.intrinsics().object_prototype());
+}
+
 // https://github.com/tc39/proposal-shadowrealm/pull/392
 static ThrowCompletionOr<NonnullGCPtr<Object>> initialize_shadow_realm(ShadowRealm& object)
 {
@@ -50,15 +58,23 @@ static ThrowCompletionOr<NonnullGCPtr<Object>> initialize_shadow_realm(ShadowRea
     // 2. Let realm be the Realm of context.
     auto& realm = *context.realm;
 
+    // FIXME: Spec bug. InitializeHostDefinedRealm is pushing on an execution context and active realm that we need to undo here.
+    //
+    // This relates to spec changes:
+    //  - https://github.com/tc39/proposal-shadowrealm/commit/76bb92682c448b8caf5cb7012c5f826be121880a
+    //  - https://github.com/tc39/proposal-shadowrealm/commit/28b0cc3f749ddcbb05290f4481bddf5b8253a4c7
+    //
+    // The proposed change to integrate with the Web Platform: https://github.com/tc39/proposal-shadowrealm/pull/392
+    //
+    // Needs to be adjusted to align with these updates.
+    vm.pop_execution_context();
+    object.set_execution_context(vm.running_execution_context().copy());
+
     // 3. Return ? HostInitializeShadowRealm(realm, context, O).
     if (vm.host_initialize_shadow_realm)
         return TRY(vm.host_initialize_shadow_realm(realm, context.copy(), object));
 
-    // AD-HOC: Fallback for when there is no host defined implementation.
-    vm.pop_execution_context();
-    object.set_execution_context(vm.running_execution_context().copy());
-    object.set_shadow_realm(*vm.running_execution_context().realm);
-    return Object::create(realm, realm.intrinsics().object_prototype());
+    return default_create_global_object(realm);
 }
 
 // 3.2.1 ShadowRealm ( ), https://tc39.es/proposal-shadowrealm/#sec-shadowrealm

Userland/Libraries/LibJS/Runtime/VM.h

@@ -280,7 +280,7 @@ class VM : public RefCounted<VM> {
     Function<ThrowCompletionOr<void>(Object&)> host_ensure_can_add_private_element;
     Function<ThrowCompletionOr<HandledByHost>(ArrayBuffer&, size_t)> host_resize_array_buffer;
     Function<void(StringView)> host_unrecognized_date_string;
-    Function<ThrowCompletionOr<NonnullGCPtr<ShadowRealm>>(Realm&, NonnullOwnPtr<ExecutionContext>, ShadowRealm&)> host_initialize_shadow_realm;
+    Function<ThrowCompletionOr<NonnullGCPtr<Object>>(Realm&, NonnullOwnPtr<ExecutionContext>, ShadowRealm&)> host_initialize_shadow_realm;
 
     Vector<StackTraceElement> stack_trace() const;