Fixed #16807

Resolves #16822

CHANGELOG.md

@@ -14,6 +14,7 @@
 - Fixed an error that could occur when saving a Commerce variant. ([#16789](https://github.com/craftcms/cms/pull/16789))
 - Fixed a bug where two entries could be created simultaneously when creating a new entry via the “New entry” menu on the Entries index page. ([#16487](https://github.com/craftcms/cms/issues/16487))
 - Fixed an error that could occur if a field was removed from a field layout, if another field had been conditional based on it. ([#16801](https://github.com/craftcms/cms/issues/16801))
+- Fixed an error that occurred when attempting to save a nested entry via a slideout, which had been duplicated from another entry and wasn’t yet fully saved. ([#16807](https://github.com/craftcms/cms/issues/16807))
 - Fixed a styling issue. ([#16791](https://github.com/craftcms/cms/issues/16791))
 
 ## 5.6.10.2 - 2025-02-26

src/helpers/Cp.php

@@ -933,6 +933,7 @@ private static function baseElementAttributes(ElementInterface $element, array $
                     'owner-id' => $element instanceof NestedElementInterface ? $element->getOwnerId() : null,
                     'owner-is-canonical' => self::elementOwnerIsCanonical($element),
                     'site-id' => $element->siteId,
+                    'is-unpublished-draft' => $element->getIsUnpublishedDraft(),
                     'status' => $element->getStatus(),
                     'label' => (string)$element,
                     'url' => $element->getUrl(),

src/web/assets/cp/src/js/NestedElementManager.js

@@ -489,6 +489,7 @@ Craft.NestedElementManager = Garnish.Base.extend(
           if (
             typeof this.elementEditor !== 'undefined' &&
             Garnish.hasAttr($element, 'data-owner-is-canonical') &&
+            !Garnish.hasAttr($element, 'data-is-unpublished-draft') &&
             !this.elementEditor.settings.isUnpublishedDraft
           ) {
             await slideout.elementEditor.checkForm(true, true);