> Hmm, maybe 'C-x t n' should not reuse the same window,
> but should create a new window with exactly the same state.
> This will avoid such problems.  Ok, I'll try to use
> window-state-put/get while cloning tabs.

Here is attempt to create new windows with 'C-x t n'.
It works fine with (window-state-put (window-state-get))
to clone all windows in the tab (from the frame window).

But 'tab-bar-move-window-to-tab' still doesn't work
when it binds 'tab-bar-new-tab-choice' to 'window'.

The problem is that this doesn't create a new window
from the selected window:

  (window-state-put (window-state-get (selected-window)) (selected-window))

Maybe Martin could help.