"Use theme CSS" gives incorrect CSS when '#groups' array doesn't line up with child elements

On a site that upgrading from 'wysiwyg' 2.2 to 'wysiwyg' 2.4, it doesn't add a particular CSS file to the WYSIWYG. I put a whole bunch of debug statements into the _wysiwyg_pre_render_styles() function to try and determine why that was.

In _wysiwyg_pre_render_styles() it has this code:

    if (isset($elements['#groups'][$child]['group']) && $elements['#groups'][$child]['group'] != CSS_THEME) {
      continue;
    }

This is assuming that the numerical indexes (ie. $child could equal '38') in $elements line up with the numerical indexes in $element['#groups']. For whatever reason, that isn't true for this particular CSS file, and walking through the arrays, for many others as well.

In my testing, this is the $elements[$child]:

    [38] => Array
        (
            [#type] => html_tag
            [#tag] => link
            [#attributes] => Array
                (
                    [type] => text/css
                    [rel] => stylesheet
                    [href] => http://site/profiles/distro/themes/distro_theme/assets/css/bootstrap-custom.css?or8e8i
                    [media] => all
                )

            [#browsers] => Array
                (
                    [!IE] => 1
                    [IE] => 1
                )

        )

But this is $elements['#groups'][$child]:

            [38] => Array
                (
                    [type] => external
                    [group] => 0
                    [every_page] => 
                    [media] => all
                    [preprocess] => 0
                    [browsers] => Array
                        (
                            [!IE] => 1
                            [IE] => 1
                        )

                    [items] => Array
                        (
                            [0] => Array
                                (
                                    [type] => external
                                    [group] => 0
                                    [weight] => 0.056
                                    [every_page] => 
                                    [media] => all
                                    [preprocess] => 0
                                    [data] => //fonts.googleapis.com/css?family=Source+Serif+Pro:400,600,700
                                    [browsers] => Array
                                        (
                                            [!IE] => 1
                                            [IE] => 1
                                        )

                                )

                        )

                )

That's clearly not the same item! The one in $elements['#groups'][38] has a 'group' of 0, but the item at $elements[38] should have a 'group' of 100 (or CSS_THEME) that would get it included (as opposed to skipped in this case).

In this case, it's off by one -- the item at $elements['#groups'][37] is the correct one. However, when I walk through both arrays, it's not always off by one, the items are close sometimes, but mixed up in various ways.

As best as I can tell, the theme isn't doing anything unusual. The CSS stylesheet that we want to be added is added thus:

stylesheets[all][]   = assets/css/bootstrap-custom.css

And the font entry is added in template.php in THEME_preprocess_html():

drupal_add_css('//fonts.googleapis.com/css?family=Source+Serif+Pro:400,600,700', array('type' => 'external'));

However, this theme is a 3rd generation sub-theme: GRANDPARENT -> PARENT -> THEME, which may be how it got all mixed up? And the 'bootstrap-custom.css' stylesheet itself is defined in the PARENT theme and overridden in the THEME itself, which could also be related to the cause?

Anyway, I guess what I'm getting to, is that there are apparently circumstances where those arrays don't line up, and so I don't think it's safe to assume that the indexes will match. I'm going to continue try and dig into a safe way to connect a style element with it's group.