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

[Juri Linkov]

>> 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 ...

Indeed, this would be an incompatible change.

So better to fix callers not to use frame-root-window
that is unreliable: sometimes frame-root-window returns
an internal window, sometimes a live window:

diff --git a/lisp/tab-bar.el b/lisp/tab-bar.el
index 917b5e496b..a7d905184b 100644
--- a/lisp/tab-bar.el
+++ b/lisp/tab-bar.el
@@ -766,7 +772,7 @@ tab-bar-select-tab
                      tab-bar-history-forward)))
 
          (ws
-          (window-state-put ws (frame-root-window (selected-frame)) 'safe)))
+          (window-state-put ws nil 'safe)))
 
         (setq tab-bar-history-omit t)