Richard Stallman <rms@gnu.org> writes:
>     I don't think this is the right thing to do in all cases.
>     This shouldn't happen when there is another frame that already has a
>     minibuffer.
>
> I don't like the idea of making one frame use another frame's
> minibuffer, as a default.  If you want to do that, you can do it
> explicitly, by specifying a different value for the `minibuffer' frame
> parameter.  But the thing Emacs does automatically is create a separate
> minibuffer frame.
>
> I just commented out an incorrect change which would have altered
> that.

When Emacs starts up under X, it automatically initializes
default-minibuffer-frame to be the initial frame.  The change I made
emulates this behaviour for display devices other than the initial
one.  Without this change, Emacs behaves inconsistently.  If the
automatic setting of default-minibuffer-frame is wrong, then I suggest
to remove its assignment from `frame-initialize' as well.

-- 
Károly