Issue #2641380 by gnuget, darketaine, Chernous_dn, dinarcon, mohit_aghera, joelpittet, hass, davidhernandez, Wim Leers, Cottser: image-widget.html.twig references a non-existing classy/image-widget library

core/themes/classy/classy.libraries.yml

@@ -56,6 +56,12 @@ forum:
     component:
       css/components/forum.css: { weight: -10 }
 
+image-widget:
+  version: VERSION
+  css:
+    component:
+      css/components/image-widget.css: {}
+
 indented:
   version: VERSION
   css:

core/themes/classy/css/components/image-widget.css

@@ -1,7 +1,19 @@
-
 /**
+ * @file
  * Image upload widget.
+ *
+ * This CSS file is not used in this theme (Classy). It was intended to be used,
+ * but due to a bug, Drupal 8 shipped with it not being used. To not break
+ * backwards compatibility, we continue to not load it in Classy. Every
+ * subtheme of Classy is encouraged to use it, by attaching the
+ * classy/image-widget asset library in their image-widget.html.twig file.
+ *
+ * @see core/themes/seven/templates/content-edit/image-widget.html.twig.
+ *
+ * @todo In Drupal 9, let core/themes/classy/templates/content-edit/image-widget.html.twig
+ * attach the classy/image-widget asset library.
  */
+
 .image-preview {
   float: left; /* LTR */
   padding: 0 10px 10px 0; /* LTR */

core/themes/classy/templates/content-edit/image-widget.html.twig

@@ -10,7 +10,6 @@
  * @see template_preprocess_image_widget()
  */
 #}
-{{ attach_library('classy/image-widget') }}
 <div{{ attributes }}>
   {% if data.preview %}
     <div class="image-preview">

core/themes/seven/templates/image-widget.html.twig

@@ -0,0 +1,12 @@
+{#
+/**
+ * @file
+ * Theme override for an image field widget.
+ *
+ * Included from Classy theme.
+ *
+ * @see template_preprocess_image_widget()
+ */
+#}
+{% include '@classy/content-edit/image-widget.html.twig' %}
+{{ attach_library('classy/image-widget') }}