bug#33532: 26.1; set-window-configuration does not restore display start

[Markus Triska <triska@metalevel.at>]

martin rudalics <rudalics@gmx.at> writes:

> and the functions mentioned there are `window-state-get' and
> `window-state-put'.

These functions have an analogous issue, which you can see with:

    (let ((buf (get-buffer-create "buf")))
      (switch-to-buffer buf)
      (erase-buffer)
      (dotimes (x 10)
        (insert (format "x is %s\n" x)))
      (recenter 3)
      (sit-for 1)
      (let ((conf (window-state-get))
            (ws (window-start))
            (str (with-current-buffer buf (buffer-string))))
        (with-current-buffer buf
          (erase-buffer)
          (insert str))
        (window-state-put conf)
        (sit-for 1)
        (message "window-start: %s vs. %s" ws (window-start))))

> So you have to tell us what you want to fix: The behavior of window
> configurations throughout one and the same Emacs session (which uses
> markers to restore positions) or when restoring a previous session
> (which relies on positions only).

If possible, please fix both. I filed the simpler case first, but both
functions are broken.  For now, I would like to concentrate on the case
that only involves variables in the same Emacs session, without saving
files to disk. I can file a separate issue for the other function.

> What is "to restore the exact same text in all buffers"?  The text at
> the time the configuration was stored in a register?

It means the buffer text that can be obtained for example with
(buffer-string), and can be restored exactly as it was at any time. The
snippet I posted illustrates this case.

> Restoring a window configuration (or state) can only use the buffer
> text as is and adjust positions accordingly.

This is what I am asking for, and what currently does not work. Please
see the snippet I posted for a reproducible test case: The buffer text
is what it was, but the start position is not restored.

> Furthermore, there is no guarantee that the same buffer will be shown
> in that window when the configuration is restored.

The fact that other guarantees are also broken is definitely
unfortunate. However, improving just the particular aspect I mentioned
would already be extremely helpful for my use case.

> If you use window states instead of configurations then this should
> work.  States store positions instead of markers.

Please see above: The snippet shows that it doesn't work. Also, I am
primarily interested in the window configuration, and the documentation
suggests that the window-start is saved (and restored).

If possible, please consider implementing this, or adjusting the
documentation so that it states what is actually guaranteed.

Thank you and all the best,
Markus