bug#26513: 25.2; pop-up-frames and *Completions* buffer

[Charles A. Roelli]

>From emacs -Q:

M-x set-variable RET pop-up-frames RET t RET
M-x global- TAB

The *Completions* buffer is opened in a new frame, but the cursor is in
the frame too, so the user has to switch back to the minibuffer to
continue typing.

How can the user ensure that the cursor goes back to the minibuffer
automatically?  Could the solution be documented somewhere (maybe in the
docstring of `pop-up-frames') or could the completion code take care of
it?

In GNU Emacs 25.2.1 (x86_64-apple-darwin10.8.0, NS appkit-1038.36 Version 
10.6.8 (Build 10K549))
 of 2017-02-22 built on gray
Windowing system distributor 'Apple', version 10.3.1038
Configured using:
 'configure --with-modules'

Configured features:
JPEG RSVG NOTIFY ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS NS MODULES