Allow pre_get_posts hooks attached after WP eCommerce's wpsc_split_the_query() to run.

wpsc-split-the-query-fix.php

<?php
/**
 * Plugin Name: WP eCommerce Query Hotfix
 * Description: Allow pre_get_posts hooks attached after wpsc_split_the_query() to run.
 */

/**
 * Fix 'pre_get_post' hooks when WP eCommerce is active.
 *
 * The wpsc_split_the_query() pre_get_posts callback attempts to run only once
 * by removing itself from the filter list within the callback. Unfortunately,
 * if there is only one hook registered at a priority, it shortens the
 * $wp_filter[ $tag ] array, causing any hooks at the next priority to be
 * skipped. Since wpsc_split_the_query() is registered at priority 8 and it's
 * not likely there are any other hooks attached at the same priority, hooks at
 * the next priority won't fire.
 *
 * To fix this, a dummy 'pre_get_posts' hook is registered at priority 8 after
 * wpsc_split_the_query() is registered, thus preventing that level of the array
 * from being removed.
 *
 * @link https://core.trac.wordpress.org/ticket/21169
 * @link https://github.com/wp-e-commerce/WP-e-Commerce/blob/d9ca9a33329f50d0bab303ca52d33f4a2271a4af/wpsc-components/theme-engine-v1/helpers/query.php#L396
 */
function wpsc_split_the_query_fix() {
	if ( class_exists( 'WP_eCommerce' ) ) {
		add_action( 'pre_get_posts', '__return_null', 8 );
	}
}
add_action( 'init', 'wpsc_split_the_query_fix' );