I am thinking this is done with a nested loop so I am afraid to get into it because I know they are complicated. I want this:
Click a category.
Sidebar shows only that category, it's children and posts that are in those. It doesn't show all of the categories or all of the posts, only the ones that are specific to that page.
We are on post 2 page:
Child1 of Category
post 2 (this is the page we are on now)
Child2 of Category
(posts of child2 category are collapsed)