Ticket #42975: 42975.diff

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

@@ -609 +609 @@
          * @since 4.9.0
          */
         public function establish_loaded_changeset() {
+                global $pagenow;
+
                 if ( empty( $this->_changeset_uuid ) ) {
                         $changeset_uuid = null;
 
@@ -637 +639 @@
                         $this->_changeset_uuid = $changeset_uuid;
                 }
 
-                $this->set_changeset_lock( $this->changeset_post_id() );
+                if ( is_admin() && 'customize.php' === $pagenow ) {
+                        $this->set_changeset_lock( $this->changeset_post_id() );
+                }
         }
 
         /**