Mediawiki 1.26.2
Try to get this working in my Sidebar using the default Vector skin.
The number of sub-pages and subcategories are correct but only a gray arrow is shown and so I can't traverse the category tree.
Interesting, for categories that have their own sub-categories, the arrow is completely gone!
Here are my LocalSettings.php parameters:
$wgUseAjax = true;
require_once "$IP/extensions/CategoryTree/CategoryTree.php";
$wgCategoryTreeSidebarRoot = "Category:Sidebar";
$wgCategoryTreeSidebarOptions['showcount'] = true;