Current: opening/closing a postbox on one of the three pages, which have them: post, page, link makes the state of the other two pages empty out
Should be: every page should keep its own list of closed boxes, and not keep them in one option
The patch abstracts keeping the state of the boxes, introducing one more level -- page. The states are now kept in option named closedpostboxes_$page.
The patch also takes out some more common javascript to post/page in another js file: slug.js Page creation page doesn't load the whole post.js file, but has its own page.js.
Finally the patch, adds nonces for the closed-postboxes admin-ajax action.