1. start a vanila emacs session
2. using scratch buffer, use follow display buffer alist configuration

(setq display-buffer-alist
          '(("\\*\\(Async Shell Command\\|Shell Command Output\\)\\*"
         (display-buffer-reuse-window display-buffer-in-side-window)
         (window-width . 0.40)
         (side . right)
         (slot . 0))))

3. split the window horizontally using `C-x 3`
4. run `M-x async-shell-command ls RET`
5. run `M-x window-toggle-side-windows`
6. run `M-x window-toggle-side-windows`

You will see that there is an extra, window added, to the frame.



In GNU Emacs 27.2 (build 1, x86_64-apple-darwin18.7.0, NS appkit-1671.60 Version 10.14.6 (Build 18G95))
of 2021-03-28 built on builder10-14.porkrind.org
Windowing system distributor 'Apple', version 10.3.2022
System Description:  macOS 11.6

Recent messages:
(No changes need to be saved)
Mark set
Quit
Mark set
Scanning for dabbrevs...done
user-error: No dynamic expansion for ‘window-toggle-side-windows`’ found
Mark set
(("\\*\\(Async Shell Command\\|Shell Command Output\\)\\*" (display-buffer-reuse-window display-buffer-in-side-window) (window-width . 0.4) (side . right) (slot . 0)))
You can run the command ‘async-shell-command’ with M-&
ls: finished.

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 KQUEUE ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS NS MODULES
THREADS JSON PDUMPER GMP

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

Major mode: Lisp Interaction

Minor modes in effect:
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  undo-tree-mode: t
  shell-dirtrack-mode: t
  ivy-mode: t
  which-key-mode: t
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  page-break-lines-mode: t
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  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
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  mouse-wheel-mode: t
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  global-font-lock-mode: t
  font-lock-mode: t
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  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
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Load-path shadows:
/Users/indrajeet.khandekar/.emacs.d/elpa/magit-3.2.1/magit-section-pkg hides /Users/indrajeet.khandekar/.emacs.d/elpa/magit-section-3.2.1/magit-section-pkg

Features:
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env code-pages mule custom widget hashtable-print-readable backquote
threads kqueue cocoa ns multi-tty make-network-process emacs)

Memory information:
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