bug#46904: Non-unique windows produced by window-state-put

[martin rudalics <rudalics@gmx.at>]

 > Actually with this patch window-swap-states fails with the error
 > "Wrong type argument: stringp, nil" in:
 >
 >        ;; Swap basic states.
 >        (window-state-put state-1 window-2 t)
 >        (window-state-put state-2 window-1 t)
 >        ;; Swap overlays with `window' property.
 >        (with-current-buffer (window-buffer window-1)
 >
 > Here '(window-buffer window-1)' returns nil because
 > window-1 doesn't exist anymore after calling window-state-put.

We could easily handle that by having `window-state-put' return the
window actually used for putting the state into and use that in
`window-swap-states'.  Yet someone else might use `window-state-put' in
much the same way as `window-swap-states' does and we have a quite
incompatible change: We nowhere say that the window used as second
argument of `window-state-put' will be live if it was so before, but we
neither say that it can be deleted.  So I'm not sure what to do ...

martin