Attached you'll find a new patch addressing all you issues.

I have integrated our discussion leading to
https://list.orgmode.org/87ply6nyue.fsf@localhost/
Please feel free to add the line

Co-authored-by: Ihor Radchenko <yantar92@posteo.net>

to the commit message.  I think you should.

On 09.01.2024 15:49, Ihor Radchenko wrote:
> gerard.vermeulen@posteo.net writes:
> 
>> Attached you'll find a new patch fixing the three wrong lines in the
>> previous
>> and now the ERT test checks also for `user-error's.
> 
> Thanks!
> 
>>> 2. Your patch does not create space between the src blocks, unlike 
>>> what
>>>    we have on main.
>>>    I think that you need to (1) create a single blank lines between
>>>    blocks (set :post-blank property to 1); (2) keep the number blank
>>>    lines after the last block the same as in the initial block (copy
>>> the
>>>    :post-blank property and assign it to the last inserted src 
>>> block).
>>> 
>>>    For C-u argument, do not do anything special - just keep the
>>> original
>>>    :post-blank as is. It is the closest to what we have on main.
>>> 
>> 
>> The previous version of the patch had
>> +            (insert (if arg (concat stars "\n") ""))
>> and now it has
>> +            (insert (if arg (concat stars "\n") "\n"))
>> I prefer this over setting the :post-blank property because it is
>> simpler.
> 
> Yet, it will lead to large spacing between src blocks in the following
> scenario:
> 
> --------------------
> #+begin_src emacs-lisp
>   "This is test"
> <point>  "This is test2"
>   "This is test3"
> #+end_src
> 
> 
> 
> 
> 
> 
> Paragraph.
> --------------------
> 

I have used the :post-blank property to fix it.  The result is now:
--------------------
#+begin_src emacs-lisp
   "This is test"
#+end_src

#+begin_src emacs-lisp
   "This is test2"
   "This is test3"
#+end_src






Paragraph.
--------------------

Here is also a redone comparison between main and patch:

------- begin comparison main and patch
#+begin_src emacs-lisp :results silent
(setopt org-adapt-indentation t
         org-edit-src-content-indentation 2
         org-src-preserve-indentation nil))
#+end_src

* main
**** C-u C-x C-v C-d
      #+CAPTION: caption.
      #+NAME: name.
      #+BEGIN_SRC emacs-lisp
        above
        ;; region
        below
      #+END_SRC
becomes
**** C-u C-x C-v C-d
      #+CAPTION: caption.
      #+NAME: name.
      #+BEGIN_SRC emacs-lisp
        above

        #+END_SRC
****
        #+BEGIN_SRC emacs-lisp
        ;; region
        #+END_SRC
****
        #+BEGIN_SRC emacs-lisp
        below
      #+END_SRC
pitfall

* patch
**** C-u C-x C-v C-d
      #+CAPTION: caption.
      #+NAME: name.
      #+BEGIN_SRC emacs-lisp
        above
        ;; region
        below
      #+END_SRC
becomes
**** C-u C-x C-v C-d
      #+caption: caption.
      #+name: name.
      #+begin_src emacs-lisp
        above
      #+end_src
****
      #+begin_src emacs-lisp
        ;; region
      #+end_src
****
      #+begin_src emacs-lisp

        below
      #+end_src
pitfall
------- end comparison main and patch

> 
> Also, your new version of the patch will completely misbehave because 
> of
> setting mark. Please, use `region-beginning' and `region-end' instead.
> Setting and checking mark is not to be done in Elisp - it only make
> sense when transient-mark-mode is enabled.
> 
Done. See below.
> 
>> * Adding a user option for properties to set to nil in 
>> org-element-copy.
>> 
>> This may be overkill, but something like:
>> 
>> #+begin_src emacs-lisp :results nil :tangle no
>> (defcustom org-babel-demarcate-block-delete '(:caption :name)
>>    "List of things to delete from blocks below the upper block when
>> splitting blocks by demarcation.  Possible things are `:caption' to
>> delete \"#+CAPTION:\" keywords, `:header' to delete \"#+HEADER:\"
>> keywords, `:name' to delete \"#+NAME:\" keywords, and `switches'
>> to delete e.g. \"-i +n 10\" from the \"#+BEGIN_SRC\" line."
>>    :group 'org-babel
>>    :package-version '(Org . "9.7")
>>    :type '(set :tag "Things to delete when splitting blocks by
>> demarcation"
>>                (const :caption)
>>                (const :header)
>>                (const :name)
>>                (const :switches))
>>    :initialize #'custom-initialize-default
>>    :set (lambda (sym val)
>>           (set-default sym val)))
>> #+end_src
> 
> That would make sense. Although, we do not have to limit the possible
> options to just what you listed. Arbitrary affiliated keywords might
> also be excluded. For example, #+ATTR_HTML keyword is stored in src
> block object as :attr_html.
> 
I prefer to postpone work on this idea.
> 
>> +          ;; To simplify the (unless ... (user-error ...)).
>> +          (unless (org-region-active-p) (set-mark (point)))
> 
> Setting mark causes issue in my above example.
> 
>> +          ;; Test mark to be more specific than "Not at a block".
>> +          (unless (and (>= (point) body-beg) (<= (mark) body-end))
>> +            (user-error "Select within the source block body to split 
>> it"))
> 
> Here, it is better to use `region-beginning', `point', and 
> `region-end'.
> `region-beginning', unlike mark and point, is guaranteed to be _before_
> `region-end'. Mark may be before point, in contrast.
> 
> You can write something like
> 
> (unless
>  (if (org-region-active-p)
>    (<= body-beg (region-beginning) (region-end) body-end)
>   (>= body-beg (point)))
>  (user-error ...))

Done, using a better initialization of parts, the test simplifies to:

            ;; Point or region are within body when parts is in 
increasing order.
            (unless (apply #'<= parts)
              (user-error "Select within the source block body to split 
it"))

Regards -- Gerard