User limited items hide/show with CSS

User limited items hide/show with CSS
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A small, but potentially useful, feature has been added to trunk.

It works around the fact that ViewCache is role based and not user based. A good
example of an old feature where it could be used are Exponential based forums:
when users have the content/edit/owner(self) policy, they must be offered a link
to edit their own posts. But since forums are cached using ViewCache, each page
is shared among all users, and if you use a content/edit policy check for each
link, you will end up with a user specific cache used by several users.

This can be worked around:
- using AJAX, but with a high performances cost
- by showing the links to all users, with a usability loss
- or by disabling ViewCache

This feature provides another workaround, that will in most cases offer the
expected user experience, with no security or performance added costs.

The page_head_displaystyles.tpl template
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This template defines two styles:

* .limitdisplay-user
  This class will completely hide any item it is applied to

*  .limitdisplay-user-<UserID>
  This class will show items that match the currently logged in user ID
  
This template is part of standard, and is included from standard, webin and flow
pagelayout.tpl.

Using it
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When a link/button should only be shown to its "owner" (the edit/owner(self))
example above, just apply two classes to it::

    class="limitdisplay-user limitdisplay-user-{$current_user.contentobject_id}"
    
The first class will hide these for everybody, while the second one will
make the item visible for its owner.

If styles aren't applied due to a specific user agent, etc, the items will be
visible for anyone, but server side security checks will still ensure full
security for your data.