bug#44933: 27.1; Ephemeral frame selection shrinks minibuffer

[Sean Devlin]

Hi folks,

Selecting a different frame ephmerally (e.g. via `with-selected-frame`
in a timer callback) shrinks the minibuffer. This is true whether or not
the minibuffer is active.

Reproduction steps:

1. Evaluate this form in the scratch buffer:

   (run-with-timer nil 13 (lambda () (with-selected-frame
   (next-frame))))

2. Evaluate this form in the scratch buffer:

   (run-with-timer nil 11 (lambda () (window-resize (minibuffer-window)
   10)))

3. Observe that the minibuffer grows and shrinks as the timers fire.

You can run some completing read command (e.g. `M-x`) to see that the
resizing happens whether or not the minibuffer is active.

This is relevant for completing read implementations that resize the
minibuffer to display a vertical list of candidates. I noticed the
behavior while using selectrum (https://github.com/raxod502/selectrum)
while I had a subprocess running under term in another frame. Since the 
`term-emulate-terminal` function selects windows in the background  
whenever the subprocess sends output, it was causing the selectrum
minibuffer to shrink.

I'm not sure what the correct behavior is here, but this was unexpected
to me. I think that if the minibuffer is active, ephemeral frame or
window selections should not affect its size. (I'm less certain about
the inactive minibuffer case, but I think the size should stay the same
there as well.)

Thanks!

In GNU Emacs 27.1 (build 1, x86_64-apple-darwin18.7.0, NS appkit-1671.60 
Version 10.14.6 (Build 18G95))
of 2020-08-12 built on builder10-14.porkrind.org
Windowing system distributor 'Apple', version 10.3.2022
System Description:  macOS 11.0

Recent messages:
Beginning of buffer [5 times]
uncompressing window.el.gz...done
Making completion list...
Type C-x 1 to delete the help window, C-M-v to scroll help.
t [2 times]
Quit [3 times]
[nil 24514 47830 650446 10 (closure (t) nil (window-resize (minibuffer-window) 
10)) nil nil 0]
[nil 24514 47883 629069 11 (closure (t) nil (let (... ...) (unwind-protect ... 
... ...))) nil nil 0]
End of buffer [6 times]
Beginning of buffer [8 times]

Configured using:
'configure --with-ns '--enable-locallisppath=/Library/Application
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Configured features:
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THREADS JSON PDUMPER

Important settings:
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Major mode: Lisp Interaction

Minor modes in effect:
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  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
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Memory information:
((conses 16 59777 30679)
(symbols 48 6538 1)
(strings 32 19388 2002)
(string-bytes 1 586736)
(vectors 16 11741)
(vector-slots 8 146822 21486)
(floats 8 40 48)
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