Source

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

protected function start_capturing_option_updates() {
		if ( $this->_is_capturing_option_updates ) {
			return;
		}

		$this->_is_capturing_option_updates = true;

		add_filter( 'pre_update_option', array( $this, 'capture_filter_pre_update_option' ), 10, 3 );
	}