Fix category query tweak + sticky posts + is_home issue.

functions.php

<?php

add_filter( 'posts_results','child_query_is_home' );
/**
 * Fixes an issue where a query that targets a taxonomy stops sticky posts from working
 * on is_home.
 *
 * The code suggests that sticky posts should only be on the blog archive (is_home) page.
 * However, if that query is amended with an inclusive cat, tag or tax restriction, then
 * WP doesn't count the query as being the blog archive page anymore.
 *
 * In wp-includes/query.php, for each tax query that is not a "NOT IN" operator, the
 * relevant is_category, is_tag, or is_tax property is set to true. This in turn means
 * that is_archive is set to true, and this means is_home is set to false. Since the
 * rearrangements of sticky posts requires is_home to be true, sticky posts aren't
 * moved to the top under these circumstances.
 *
 * This function doesn't actually filter anything, but uses the raw results filter hook
 * to jump in and set is_home back to true on the global $wp_query so that sticky posts
 * start appearing first again. If you have multiple queries on a page, you may want to
 * set conditions under which this tweak is made.
 *
 * @author Gary Jones
 * @link   https://gist.github.com/1950278
 *
 * @global WP_Query $wp_query Posts query object.
 *
 * @param array $args Raw results array of posts and current WP_Query object.
 *
 * @return array Original untouched $args.
 */
function child_query_is_home( array $args ) {
	global $wp_query;
	$wp_query->is_home = true;
	return $args;
}