Category-Post AJAX Tree

Add a sidebar widget that displays expandable categories menu.
Widget shows only 2 level menu – no subcategories just posts in categories.
Two available styles. Easy to understand styling.
Post list is fetched using AJAX calls upon request.
If current page is a post or category with posts corresponding menu is loaded and expanded.
If post belongs in more than one category only its first category menu is expanded.
There are two styles currently. One has arrow images displayed permanently showing upwards
or downwards depending on menu being opened/closed

Arbitrary section

A brief Markdown Example