Before `window-configuration-to-register'

 +-------------------+-----------------+
 |                   |                 |
 |                   |                 |
 |                   |                 |
 |                   |                 |
 +-------------------+-----------------+

 After `window-configuration-to-register'

 +-------------------------------------+
 |                                     |
 |                                     |
 |                                     |
 |                                     |
 +-------------------------------------+

 Then last (wrong) window configuration is saved to register.

 ```
 (defun register-read-with-preview (prompt)
 ...
      (let ((w (get-buffer-window buffer)))
        (and (window-live-p w) (delete-window w)))   <=== ### Don't check
 pre existing window ###
      (and (get-buffer buffer) (kill-buffer buffer)))))
 ```

 Thanks. Best regards.


 In GNU Emacs 27.0.91 (build 4, x86_64-pc-linux-gnu, cairo version 1.16.0)
 of 2020-05-06 built on Ubuntu-18
 Repository revision: f8e6cd11b34320999aa72886ece3f32b0a8ad897
 Repository branch: emacs-27
 Windowing system distributor 'The X.Org Foundation', version 11.0.12005000
 System Description: Ubuntu 19.10

 Configured using:
 'configure --with-cairo --with-x-toolkit=no CFLAGS=-O3'

 Configured features:
 XPM JPEG TIFF GIF PNG CAIRO SOUND GPM DBUS GSETTINGS GLIB NOTIFY INOTIFY
 ACL LIBSELINUX GNUTLS LIBXML2 FREETYPE HARFBUZZ ZLIB OLDXMENU X11 XDBE
 XIM MODULES THREADS PDUMPER GMP

 Important settings:
  value of $LC_ALL:
  value of $LANG: es_ES.UTF-8
  locale-coding-system: utf-8
Date: Wed, 06 May 2020 13:11:21 +0200
Message-ID:
<87zhalgvxi.fsf@Ubuntu-18.i-did-not-set--mail-host-address--so-tickle-me>