bug#72331: 29.4; Modifying horizontal-scroll-bar does not fully work (MacOS only)

[Jared Finder]

Directly modifying the variable horizontal-scroll-bar on MacOS doesn't
show a scroll bar unless the frame also has the horizontal-scroll-bars
frame  parameter set.  This only happens on MacOS, on Linux and Windows
everything behaves as I would expect.

To reproduce, define the following function:

(defun bug-report-toggle-buffer-horizontal-scroll-bar ()
  "Like `toggle-scroll-bar', but for just the current buffer.
And for horizontal scroll bars.  I guess it's acutally very
different."
  (interactive)
  (setq horizontal-scroll-bar (if horizontal-scroll-bar nil 'bottom))
  (set-window-buffer (selected-window) (current-buffer)))

Then run M-x bug-report-toggle-buffer-horizontal-scroll-bar repeatedly
and observe that on MacOS space gets reserved for the horizontal scroll
bar but no scroll bar is rendered.  Finally, run M-x
horizontal-scroll-bar-mode and observe that now the horizontal scroll
bar is properly shown and hidden.

I also tested this on master and observed the same behavior.

In GNU Emacs 29.4 (build 1, aarch64-apple-darwin21.6.0, NS
 appkit-2113.60 Version 12.6.6 (Build 21G646)) of 2024-07-24 built on
 armbob.lan
Windowing system distributor 'Apple', version 10.3.2487
System Description:  macOS 14.5

Configured using:
 'configure --with-ns '--enable-locallisppath=/Library/Application
 Support/Emacs/${version}/site-lisp:/Library/Application
 Support/Emacs/site-lisp' --with-modules 'CFLAGS=-DFD_SETSIZE=10000
 -DDARWIN_UNLIMITED_SELECT' --with-x-toolkit=no'

Configured features:
ACL GLIB GMP GNUTLS JPEG JSON LIBXML2 MODULES NOTIFY KQUEUE NS PDUMPER
PNG RSVG SQLITE3 THREADS TIFF TOOLKIT_SCROLL_BARS TREE_SITTER ZLIB

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

Major mode: ELisp/l

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  show-paren-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
  line-number-mode: t
  indent-tabs-mode: t
  transient-mark-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr vc-git diff-mode easy-mmode vc-dispatcher
cl-extra cl-print byte-opt gv bytecomp byte-compile thingatpt help-fns
radix-tree help-mode dabbrev emacsbug message mailcap yank-media puny
dired dired-loaddefs rfc822 mml mml-sec password-cache epa derived epg
rfc6068 epg-config gnus-util text-property-search time-date subr-x
mm-decode mm-bodies mm-encode mail-parse rfc2231 mailabbrev gmm-utils
mailheader cl-loaddefs cl-lib sendmail rfc2047 rfc2045 ietf-drums
mm-util mail-prsvr mail-utils rmc iso-transl tooltip cconv eldoc paren
electric uniquify ediff-hook vc-hooks lisp-float-type elisp-mode 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 lisp-mode prog-mode register page tab-bar menu-bar rfn-eshadow
isearch easymenu timer select scroll-bar mouse jit-lock font-lock syntax
font-core term/tty-colors frame minibuffer nadvice seq simple cl-generic
indonesian philippine 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 emoji-zwj charscript charprop case-table epa-hook
jka-cmpr-hook help abbrev obarray oclosure cl-preloaded button loaddefs
theme-loaddefs faces cus-face macroexp files window text-properties
overlay sha1 md5 base64 format env code-pages mule custom widget keymap
hashtable-print-readable backquote threads kqueue cocoa ns multi-tty
make-network-process emacs)

Memory information:
((conses 16 60165 20431)
 (symbols 48 6527 0)
 (strings 32 20561 2286)
 (string-bytes 1 561629)
 (vectors 16 13714)
 (vector-slots 8 194672 14038)
 (floats 8 33 135)
 (intervals 56 399 17)
 (buffers 984 14))