Customize: Prevent frontend previewing from causing a customization lock on the changeset.

Props dlh.
Amends [41839].
See #42024.
Fixes #42975 for 4.9 branch.

Built from https://develop.svn.wordpress.org/branches/4.9@42542


git-svn-id: http://core.svn.wordpress.org/branches/4.9@42371 1a063a9b-81f0-0310-95a4-ce76da25c4cd

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

@@ -607,8 +607,11 @@ final class WP_Customize_Manager {
 	 * enabled, then a new UUID will be generated.
 	 *
 	 * @since 4.9.0
+	 * @global string $pagenow
 	 */
 	public function establish_loaded_changeset() {
+		global $pagenow;
+
 		if ( empty( $this->_changeset_uuid ) ) {
 			$changeset_uuid = null;
 
@@ -635,8 +638,10 @@ final class WP_Customize_Manager {
 			$this->_changeset_uuid = $changeset_uuid;
 		}
 
+		if ( is_admin() && 'customize.php' === $pagenow ) {
 			$this->set_changeset_lock( $this->changeset_post_id() );
 		}
+	}
 
 	/**
 	 * Callback to validate a theme once it is loaded

wp-includes/version.php

@@ -4,7 +4,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '4.9.3-alpha-42539';
+$wp_version = '4.9.3-alpha-42542';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.