Tag Cloud Filter to Show Current Tags

Wouldn’t it be nice to see the current tags reflected in the tag cloud widget?
WordPress does not output any css classes for the current tags – as it would do for instance in the category or page menu.

A few lines added to functions.php of the currently used theme can change that:

add_filter ( 'wp_tag_cloud', 'tag_cloud_current_tag_highlight' );
function tag_cloud_current_tag_highlight( $return ) {
  $post_tags = array();
  if(is_single()) {
    global $post;
    $post_tags = get_the_terms($post->ID,'post_tag');
  }
  if(is_tag()) {
  	$tags = explode( '+', get_query_var('tag') );
	foreach( $tags as $tag ) { $post_tags[] = get_term_by('slug',$tag,'post_tag'); }
  }
  if( $post_tags ) {
	  foreach ($post_tags as $pt) {
      $tag = $pt->term_id;
        if(preg_match("#-link-" . $tag . "' #", $return)) {
        $return = str_replace("link-" . $tag . "' ", "link-" . $tag . " current-tag' ", $return);
        }
      }
	}
return $return;
}

With some formatting in style.css like the following, the current tags of the tag archive or the viewed single post can be highlighted:

.current-tag { font-style: italic; }

See the effect in the tag cloud lower down in the left sidebar of this site.