When trying to attach/link a board to an activity in my Plan, the boards are shown in a drop down-menu. However, they don't appear to be sorted alphabetically, but rather by order of creation (which isn't very helpful). Also, the boards that I've hidden show up just like all the others - great way to get real confused.
Thanks for letting us know! Well, this actually also happened to me in Chrome but you simply need to clear the browser cookies and cache and try again. What will happen is that you will then get the boards sorted in alphabetical order and the hidden boards will be shown at the bottom of the page. However I can agree that non-existing difference between hidden and active boards could be better. This will be brought up as a suggestion of improvement.