Preview fails / admin bar missing on multisite installs

[multimule]

Given a multisite install at example.org one may create one blog per sub-domain. E.g.

• foo.example.org
• bar.example.org
• whatever.example.org

Now there's the option to map additional domains to that blog (One may configure foo.com as primary domain to reach the blog at foo.example.org).

Now the backend is still located at the original domain (​http://foo.example.org/wp-admin/), but when the user _previews_ a post the following domain is called: ​http://foo.com/?p=123&preview=true

BUT this fails with a 404 error! (as the user is logged in to foo.example.org, but not to foo.org)

It'd be good to have authentication work for mapped domains too, as that would apparently solve the problem.

Of course one may switch even the backend to the mapped domain, but then users/admins of the multisite install would have to log into each and every blog separately when switching between them - effectively destroying one of the main attractive features of a multisite install (see also: ​http://wordpress.stackexchange.com/questions/17272/remote-login-with-multisite-domain-mapping-still-forcing-users-to-login-to-all)

Note that this is related to bug #16776 which was falsely considered a plugin-related issue and thus closed some months ago.

Could you explain how this pertains to core and not the domain mapping plugin?

I am missing the in-depth knowledge about the authentication specifics. Thus I am unsure if core should support this or if a plugin is at fault ...

As a follow-up, I've pointed out this issue in the wordpress-mu-domain-mapping forum/tag: ​https://wordpress.org/support/topic/post-preview-does-not-work?replies=9#post-2226380

Maybe the plugin devs are able to assess the situation.

Closing as this is explicitly an issue with the domain mapping plugin.