>> >> > 09) close other tabs with 'C-x t 1'
>> >> > 10) change window layout with 'C-x 1' and 'C-x <left>'
>> >> > 11) restore frameset with 'C-x r j a'
>> >> > 12) BUG: it shows 03 tabs, must should show only 1
>> >> 
>> >> We need to clarify the semantics of 'C-x r f':
>> >> what it's expected for it to do with tabs?
>> >
>> > Theoretically, tabs are not part of a frameset, so I think the
>> > expectations here are incorrect.
>> 
>> However, for example, in the desktop file tabs are part of a frameset,
>> and tabs are restored by restoring the frameset from the desktop file.
>
> I'm confused: if framesets support tabs and their restoration, then
> why did you say that we need to clarify what "C-x r f" does with tabs?

I don't use "C-x r f" and didn't know what it does,
so needed to look at 'frameset-to-register',
and surprisingly it uses the same 'frameset-save'
that is used by the desktop.  

But the problem is that its filters didn't use 'copy-tree'
for tabs, so any changes in tabs within the current session
also modified the data saved in the register too.

I didn't find a suitable existing filter in 'frameset-filter-params'
that uses 'copy-tree' on saving, so added a new filter ':copy-tree':