As pointed out at #484820: Initial D7UX admin header, the added page_top region was used for the admin toolbar, but it is not part of block positioning, so putting it into an actual region does not have much of a point. Blocks cannot be put above and below it and the admin cannot use blocks administration to move the toolbar around. We either need to make the toolbar an actual block or get rid of the region and make the toolbar a hook_footer() implementation. @sun and @catch suggested the latter, so I've opened this issue for that. Patch coming up.