Ticket #38864: 38864.0.diff

src/wp-includes/class-wp-customize-setting.php

@@ -696 +696 @@
                 $is_core_type = ( 'option' === $this->type || 'theme_mod' === $this->type );
 
                 if ( ! $is_core_type && ! $this->is_multidimensional_aggregated ) {
+
+                        // Use post value if previewed and a post value is present.
+                        if ( $this->is_previewed ) {
+                                $value = $this->post_value( null );
+                                if ( null !== $value ) {
+                                        return $value;
+                                }
+                        }
+
                         $value = $this->get_root_value( $this->default );
 
                         /**

src/wp-includes/customize/class-wp-customize-custom-css-setting.php

@@ -132 +132 @@
          * @return string
          */
         public function value() {
-                $id_base = $this->id_data['base'];
-                if ( $this->is_previewed && null !== $this->post_value( null ) ) {
-                        return $this->post_value();
+                if ( $this->is_previewed ) {
+                        $post_value = $this->post_value( null );
+                        if ( null !== $post_value ) {
+                                return $post_value;
+                        }
                 }
+                $id_base = $this->id_data['base'];
                 $value = '';
                 $post = wp_get_custom_css_post( $this->stylesheet );
                 if ( $post ) {