// Discussion in devel.
https://lists.gnu.org/archive/html/emacs-devel/2012-12/msg00521.html
Short:
I want to be able to set buffer that will be displayed in window of newly
created frame.
I need it for my emacs package, that will save and restore some emacs state
including
last window configurations, opened and displayed buffers and such.
I tried this: add hook to `after-make-frame-functions'. Inside this hook I
do
`(switch-to-buffer <some-buffer>)' and frame is switched to that
<some-buffer> at first,
but after a short time it's switched to *scratch*.

The one way I found how to overcome this behaviour is by setting the
`initial-buffer-choice' in the `after-make-frame-functions' hook. One
drawback of this
approach is that it could only be set to name of file, t or nil and so
there is no way to
force switching to buffer without underlying file. This could be fixed int
server.el:1258
replacing

  (unless (or files commands)
        (if (stringp initial-buffer-choice)
        (find-file initial-buffer-choice)
          (switch-to-buffer (get-buffer-create "*scratch*")
                'norecord)))

to
          (unless (or files commands)
            (switch-to-buffer
             (get-buffer-create
              (or (cond
                   ((stringp initial-buffer-choice) (find-file-noselect
initial-buffer-choice))
                   ((functionp initial-buffer-choice) (funcall
initial-buffer-choice)))
                  "*scratch*"))
             'norecord))

and modified defcustom in in startup.el:41 would be:

(defcustom initial-buffer-choice nil
  "Buffer to show after starting Emacs.
If the value is nil and `inhibit-startup-screen' is nil, show the
startup screen.  If the value is a string, visit the specified file
or directory using `find-file'.  If t, open the `*scratch*'
buffer. If function, switch to a buffer returned by this function.

A string value also causes emacsclient to open the specified file
or directory when no target file is specified."
  :type '(choice
      (const     :tag "Startup screen" nil)
      (directory :tag "Directory" :value "~/")
      (file      :tag "File" :value "~/.emacs")
      (function  :tag "Function")
      (const     :tag "Lisp scratch buffer" t))
  :version "23.1"
  :group 'initialization)

This code allows setting `initial-buffer-choice' to a function returning
needed buffer.