Cygnet styles leaking into CKEditor 5 editor

Turns out that Drupal core already provides a feature to add "donut scoping" to stylesheets, so that those stylesheets don't target CKEditor 5 instances (see offCanvasCss in ckeditor5.es6.js). However, this method is not consistently applying to stylesheets for Patternkit's JSON Editor (JE) themes (in a large Drupal 9 site, this feature is working, but in a small D9 site, it is not). I believe the problem is due to a race condition, where PK adds stylesheets to the DOM, but those stylesheets are not in the `document.styleSheets` list.

I think we should re-consider how we add JE theme assets to the DOM. Instead of using JS to add the stylesheets, why not attach them as regular Drupal libraries?

Here's a change that seems to work ok for the Cygnet JE theme, in my very limited testing:

diff --git a/js/patternkit.jsoneditor.js b/js/patternkit.jsoneditor.js
index 9bbbefc..42a2fb6 100644
--- a/js/patternkit.jsoneditor.js
+++ b/js/patternkit.jsoneditor.js
@@ -1410,17 +1410,6 @@ function _createClass(Constructor, protoProps, staticProps) { if (protoProps) _d
 
         var editor_dom = '';
 
-        if (this.settings.themeStylesheet) {
-          var theme_element = document.createElement('link');
-          theme_element.rel = "stylesheet";
-          theme_element.id = "theme_stylesheet";
-          theme_element.href = this.settings.themeStylesheet;
-          document.getElementsByTagName('head')[0].appendChild(theme_element);
-          editor_dom = '<link rel="stylesheet" id="theme_stylesheet" href="' + this.settings.themeStylesheet + '">';
-        } // @todo Re-eval with this shadow dom webfont bug:
-        // https://bugs.chromium.org/p/chromium/issues/detail?id=336876
-
-
         if (this.settings.iconStylesheet) {
           var icons_element = document.createElement('link');
           icons_element.rel = "stylesheet";
diff --git a/src/JSONSchemaEditorTrait.php b/src/JSONSchemaEditorTrait.php
index ac2e45d..512d29e 100644
--- a/src/JSONSchemaEditorTrait.php
+++ b/src/JSONSchemaEditorTrait.php
@@ -114,15 +114,6 @@ trait JSONSchemaEditorTrait {
 
     if (isset(PatternLibraryJSONForm::THEMES[$theme])) {
       $theme_info = PatternLibraryJSONForm::THEMES[$theme];
-      if (!$editor_settings['useShadowDom'] && !empty($theme_info['css_no_shadow_dom'])) {
-        $editor_settings['themeStylesheet'] = $this->getLibraryAssetUrlFromUri($theme_info['css_no_shadow_dom']);
-      }
-      elseif (!empty($theme_info['css'])) {
-        $editor_settings['themeStylesheet'] = $this->getLibraryAssetUrlFromUri($theme_info['css']);
-      }
-      else {
-        $editor_settings['themeStylesheet'] = '';
-      }
       if (!empty($theme_info['js'])) {
         $editor_settings['themeJS'][] = $this->getLibraryAssetUrlFromUri($theme_info['js']);
       }
@@ -196,6 +187,13 @@ trait JSONSchemaEditorTrait {
       ],
     ];
 
+    if (!$editor_settings['useShadowDom'] && !empty($theme_info['css_no_shadow_dom'])) {
+      $element['#attached']['library'][] = 'patternkit/cygnet_no_shadow_dom';
+    }
+    elseif (!empty($theme_info['css'])) {
+      $element['#attached']['library'][] = 'patternkit/cygnet';
+    }
+
     if (!empty($editor)) {
       $element['#attached']['library'][] = 'editor/drupal.editor';

An alternative approach is to delay loading CKEditor until after the JSON Editor (JE) theme assets have been loaded. Here's a patch that tries to do this. This would be a quicker fix than loading the JE theme assets via Drupal's library system.

To fix this, maybe we could load the JE theme assets via Drupal's library system, instead of manually injecting them by JS.

The change you mentioned here has been documented as a follow-up improvement to address via 📌 Attach JSON Editor Theme Assets Using Library Attachments Postponed . I'm relating the tickets now to document that better.