Some people mention how Wayland does not allow access to other windows from other clients, and is sometimes good and bad. In my X12 idea, this is independent of the protocol, and the protocol doesn't care. Restricted access may be implemented either by the X server itself or by some proxy, and either way the protocol doesn't care and the client doesn't care. What the restrictions are may also be implementation-dependent and is preferably configurable by the user. The client would send a SCM_CREDENTIALS message to the server; if the server doesn't understand it then it will just ignore it and allow (or deny) access anyways. (It isn't necessarily only other windows that you may restrict access to; you might also sandbox selections in a proxy too, if you want to.) (Note that an implementation which cannot be configured for unrestricted access technically is not compliant with the specification; although merely allowing it to be configured with restrictions does not violate the specification.)
