Module breaks threaded comment display

With the Comment Display module enabled, I found that the threaded comment display failed to indent replies properly. Further investigation revealed that this was because comment.css was not being included in the page headers.

Apparently, the core issue was that the drupal_add_css function (normally called from comment_render) does not function properly when called within a _preprocess_page callback, as is done under Comment Display's module. I was able to resolve this by adding the lines:

  if (function_exists('comment_render') && $node->comment) {
    drupal_add_css(drupal_get_path('module', 'comment') .'/comment.css');
  }

following the comment

// Note: Output of comments moved into comment_display_preprocess_page().

in comment_display_node_show.