bug#51007: 27.2; emacs hangs when using window-toggle-side-windows

[martin rudalics]

> 1. start vanila, emacs session.
> 2. add following configuration.
> (setq debug-on-quit 1)
> (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))
>           ("\\*Shell\\*"
>           (display-buffer-reuse-window display-buffer-in-side-window)
>           (window-width . 0.40)
>           (side . top)
>           (slot . 0))
>          )
>         )
>
> 3. split the window horizontally `C-x 3`
> 4. start a shell `M-x shell RET`
> 5. Run a async shell `M-x async-command-shell ls RET`
> 6. Run `M-x window-toggle-side-windows RET` the session stalls nothing works.
>
> - C-g does not help in this case.
> - C-x C-z does not help.
> - C-x C-c does not help.
>
>
> 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

I can't try your recipe because `async-command-shell' isn't defined
here.  When I try with

(setq debug-on-quit 1)
(setq display-buffer-alist
       '(
         ("\\*Info\\*"
         (display-buffer-reuse-window display-buffer-in-side-window)
         (window-width . 0.40)
         (side . right)
         (slot . 0))
         ("\\*Help\\*"
         (display-buffer-reuse-window display-buffer-in-side-window)
         (window-height . 0.40)
         (side . top)
         (slot . 0))
        )
       )

everything works as expected so the problem seems that shell command
output wants to use the window used for ls, or vice-versa.

martin