bug#16793: window-state-put needs a live window

[Juanma Barranquero <lekktu@gmail.com>]

Package: emacs
Version: 24.3.50
X-Debbugs-Cc: rudalics@gmx.at

There's a problem when restoring a frameset where "put"ing a window
state into the root window of a frame fails for some windows if the
frame is split (which is common enough).

It's easy to see the problem with

  emacs -Q
  C-x 2
  C-x r f a
  C-x r j a
  => "Error (frameset): #<window 6> is not a live window"

or

  emacs -Q
  ;; eval this in *scratch*
  (let (ws frw)
    (split-window)
    (setq frw (frame-root-window))
    (setq ws (window-state-get frw))
    (window-state-put ws frw))
  => "Lisp error: (error "#<window 6> is not a live window")"

Strictly speaking It's not a bug, because the docstring of
`window-state-put' says "Optional argument WINDOW must specify a live
window". But as the examples above show, it is less than useful to be
able to get the state of a window and not being able to put it again
into the same window.

In framesets, the problem arises because it's often the case that the
preferred frame to restore a window-state is the very same one the
window-state was saved from, particularly for saving/restoring in the
same session (as it happens with frameset-to-register /
jump-to-register). That minimizes flicker and simplifies dealing with
non-reused frames.

Martin suggests that frameset--restore should check that the root
window of the matching frame found is alive, and if not, discard that
frame and create a new one. That's doable, but it seems less than
optimal, from both a coding and UI perspective.