Eli Zaretskii wrote:

>> (x-set-selection 'PRIMARY value)
>> ;should just stash value somewhere
> 
> What would be the purpose of stashing the value ``somewhere''? 

The purpose is to emulate internally to emacs a primary selection 
facility otherwise missing from the platform in question. w32 emacs 
looks to have been treating the primary selection that way for the past 
13 years or so.
http://cvs.savannah.gnu.org/viewvc/emacs/lisp/w32-fns.el?revision=1.14&root=emacs&view=markup

On recent w32 it might now be possible to use an accessibility API (at 
least with sufficient privileges) to record  selections in some other 
applications, which might allow a closer to complete emulation of at 
least incoming primary, though it might be tricksy to actually implement 
and not especially worth it.

> The
> NEWS entry says:
> 
>     *** If `select-active-regions' is t, setting the mark automatically
>     makes the new region into the primary selection (for interaction with
>     other window applications).
>

Uh-huh. Note how it also says "makes the new region into the primary 
selection"  ?

> How can Emacs interact with other applications, if we just stash the
> value in some internal variable?
> 

Not all platforms have a primary selection as an OS-level entity in the 
first place, they only have a clipboard (well, the amiga actually had 
hundreds of distinct clipboards, but I'm not sure amiga support is worth 
worrying about even though AROS now exists), so emacs just has to 
interact with them via means other than a primary selection.  But mixing 
up primary and clipboard within 
x-set-selection/x-get-selection/x-selection-owner-p is wrong not
to mention phenomenally confusing.

Waht's more, I suspect it also Just Won't Work Anyway, because on w32 
AFAIK you set the system clipboard, on x11 you acquire the clipboard 
selection and applications ask you for the clipboard contents.  Perhaps 
just regard select-active-regions as a feature aimed mostly at X11 users...

If you do want select-active-regions to be able to affect the clipboard, 
a relatively sane way to do it would be to introduce customization 
variables:

x-select-active-regions-enable-clipboard 
x-select-active-regions-enable-primary

by analogy with x-select-enable-primary and x-select-enable-clipboard,
that would allow customization of which selections to actually affect 
during select-active-regions.   Note that x-select-enable-primary and 
x-select-enable-clipboard would be completely UNsuitable for reuse 
there, if you don't see why you probably haven't quite understood the issue.

See attached (illstrative) patch.