I don't understand how a group can access a nested category, and I think there are some bugs there.
I created a category and another which is nested. The main category can be seen for a special group only, and the subcategory doesn't have an option to modify it.
It's ok, logical, as the subcategory has to have the same access rights as the parent category.
A user with access (who is in the group sees the categories (on the right) and content, too.
It's ok, too.
But, when the user has no access, they see only the subcategory in the right, and the content part says this category is empty, which is not true. This is not empty, but he can't see this. What's more, there is the button to write post, and user even can write a whole post, only when publishing there comes a message "You don't have access for this source..."
BUT, what is the worst, the post is actually published in the community. (I had to remove it manually.)
In my opinion it would be better if the user saw all the category in the right panel, and clicking on them they should see: "This is a private category, and you don't have access" or something similar."
And of course the "Write a post" button shouldn't be shown, and the post mustn't be published without rights.