bug#35161: 26.1; follow-mode spurious motion

[Devon Sean McCullough]

bash$ /Applications/Emacs.app/Contents/MacOS/Emacs -Q -eval '(progn 
(insert-file-contents-literally "/dev/stdin") (eval-buffer))' <<.
(xref-find-definitions 'set-frame-font)
(toggle-frame-maximized)
(follow-mode)
(delete-other-windows)
(mark-defun)
(split-window-right)
(setq s1 0
      t1 (run-at-time 1 1 (lambda (&rest args)
                            (other-window 1)
                            (sit-for 0)
                            (when (> (setq s1 (1+ s1)) 3)
                              (cancel-timer t1)
                              (message "Now when you type C-x o the unselected 
cursor will spuriously move to the bottom line.")))))
.
bash$ ## Both windows' cursors should stay put, but instead one of them moves.

                Peace
                        --Devon

P.S. I'm curious why programmatic (other-window 1) (sit-for 0)
doesn't exhibit this bug.
Perhaps ELisp manual chapter 28.21 Windows and Point
should mention relevant pitfalls?

In GNU Emacs 26.1 (build 1, x86_64-apple-darwin14.5.0, NS appkit-1348.17 
Version 10.10.5 (Build 14F2511))
 of 2018-05-30 built on builder10-10.porkrind.org
Windowing system distributor 'Apple', version 10.3.1404
Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
uncompressing frame.el.gz...done
Mark set [2 times]
Now when you type C-x o the unselected cursor will spuriously move to the 
bottom line.

Configured using:
 'configure --with-ns '--enable-locallisppath=/Library/Application
 Support/Emacs/${version}/site-lisp:/Library/Application
 Support/Emacs/site-lisp' --with-modules'

Configured features:
NOTIFY ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS NS MODULES THREADS

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Emacs-Lisp

Minor modes in effect:
  follow-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  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:
(shadow sort mail-extr emacsbug message rmc puny seq dired
dired-loaddefs format-spec rfc822 mml mml-sec password-cache epa derived
epg epg-config gnus-util rmail rmail-loaddefs mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail
rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils follow pulse
jka-compr help-fns radix-tree help-mode easymenu find-func xref cl-seq
project ring eieio byte-opt bytecomp byte-compile cconv eieio-core
cl-macs gv eieio-loaddefs cl-loaddefs cl-lib elec-pair time-date tooltip
eldoc electric uniquify ediff-hook vc-hooks lisp-float-type mwheel
term/ns-win ns-win ucs-normalize mule-util term/common-win tool-bar dnd
fontset image regexp-opt fringe tabulated-list replace newcomment
text-mode elisp-mode lisp-mode prog-mode register page menu-bar
rfn-eshadow isearch timer select scroll-bar mouse jit-lock font-lock
syntax facemenu font-core term/tty-colors frame cl-generic cham georgian
utf-8-lang misc-lang vietnamese tibetan thai tai-viet lao korean
japanese eucjp-ms cp51932 hebrew greek romanian slovak czech european
ethiopic indian cyrillic chinese composite charscript charprop
case-table epa-hook jka-cmpr-hook help simple abbrev obarray minibuffer
cl-preloaded nadvice loaddefs button faces cus-face macroexp files
text-properties overlay sha1 md5 base64 format env code-pages mule
custom widget hashtable-print-readable backquote kqueue cocoa ns
multi-tty make-network-process emacs)

Memory information:
((conses 16 213238 12101)
 (symbols 48 21064 1)
 (miscs 40 50 228)
 (strings 32 31333 1806)
 (string-bytes 1 844158)
 (vectors 16 36768)
 (vector-slots 8 740380 12142)
 (floats 8 50 379)
 (intervals 56 575 60)
 (buffers 992 12))