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<?php
/** adds the post_id to a variable passed during refresh **/
add_action( 'wp_footer', function() {
    if ( is_singular() ) : ?>
        <script>
        (function($) {
            $(document).on('facetwp-refresh', function() {
                FWP_HTTP.post_id = <?php echo get_the_ID(); ?>
            });
        })(jQuery);
        </script>
    <?php endif;
}, 100 );

<?php
/** access post_id in query args of facetwp template settings
 ** if using a visual query you'll first need to conver that to 
 ** advanced query args **/

$post_id = isset( $this->http_params['post_id'] ) ? $this->http_params['post_id'] : 0; // OR use if that doesn't work as expected - FWP()->facet->http_params['get'][ $post_id ]
if ( $post_id < 1 ) {
    $post_id = get_the_id();
}