sidebar categories could collapse and expand

I love the fixed sidebar with the number of feeds grayed out. I'll offer another suggestion. Right now, when we click the category, I loose the "navigation always possible" usability guideline. What I do right now is to click on the "Fun" category, read them all, then press "g h" to go back to the full feed view. I'm usually a sucker for keyboard navigation, but if I'm looking at the unread post count on a number of feeds, I may just click on the category to save myself the trouble.

One way to fix it is just to collapse the other categories, and expand the clicked one. Clicking again would collapse it again. By expanding I mean have all the feeds' posts in the category shown, and to have the entire feed list of that category shown on the sidebar (with the unread post count on the right, as usual). Collapsing is to show the current screen I get with "g h".

A plus/minus sign, or a different font/colour/indentation/... could give visual clue of that expansion. This way we could always click on the same region of the screen, follow the same pattern to check all feeds.

I don't know if I'm making myself clear enough, please, feel free to ask for clarifications.