Source

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

public function establish_loaded_changeset() {
		global $pagenow;

		if ( empty( $this->_changeset_uuid ) ) {
			$changeset_uuid = null;

			if ( ! $this->branching() && $this->is_theme_active() ) {
				$unpublished_changeset_posts = $this->get_changeset_posts(
					array(
						'post_status'               => array_diff( get_post_stati(), array( 'auto-draft', 'publish', 'trash', 'inherit', 'private' ) ),
						'exclude_restore_dismissed' => false,
						'author'                    => 'any',
						'posts_per_page'            => 1,
						'order'                     => 'DESC',
						'orderby'                   => 'date',
					)
				);
				$unpublished_changeset_post  = array_shift( $unpublished_changeset_posts );
				if ( ! empty( $unpublished_changeset_post ) && wp_is_uuid( $unpublished_changeset_post->post_name ) ) {
					$changeset_uuid = $unpublished_changeset_post->post_name;
				}
			}

			// If no changeset UUID has been set yet, then generate a new one.
			if ( empty( $changeset_uuid ) ) {
				$changeset_uuid = wp_generate_uuid4();
			}

			$this->_changeset_uuid = $changeset_uuid;
		}

		if ( is_admin() && 'customize.php' === $pagenow ) {
			$this->set_changeset_lock( $this->changeset_post_id() );
		}
	}