>> I don't know how severe are the consequences.  The only side-effect I noticed
>> that prev-buffers of restored windows are the same on different tabs because
>> they share the same window.
>
> If that is a problem and the above would help, let's do it.  It couldn't
> possibly harm unless the root window is too small to get split and we
> want to put only one window there; so we should handle that special case.

The original problem was that prev-buffers of restored windows are the
same on different tabs.  This was fixed by always creating a new window.
However, the same problem still exists, but for a different reason.

Most windows have current buffer duplicated in prev-buffers.  So when
set-window-prev-buffers in window--state-put-2 sets prev-buffers to a list
with only current buffer, this means as if there are no prev-buffers.

But when a window has an empty list of prev-buffers, then the window
parameter of prev-buffers is not cleared, and still contains a random
buffer from some previous state.  This patch fixes it: