One of the features of my plugin Posts in Sidebar is that the widget can display a different content when viewing a single post page (i.e. a blog post page). For example, a user could want to display a list of recent posts in the main widget but, when viewing a single post page, a list of posts published under the same category of the main post. The feature is available under Getting posts > Change the query when on single posts.
In a recent release of my plugin Posts in Sidebar (namely the 3.8.4 version) I made an important change regarding which category to use in order to get the posts, when a post has multiple categories. In the previous versions the plugin picked up the category with the lowest letter in the alphabetically sorted list of the post’s categories and used it to get the posts. Starting from 3.8.4 the plugin can use the category with the lowest ID, that is the WordPress-style to choose the category for the permalink construction. Let’s see something more detailed on this.