Support nested lists with the AdjacentListsSupport plugin

[star-szr]

📝 Provide a description of the new feature

In #13945 it mentions that the initial scope of AdjacentListsSupport does not support nested lists.

For the project I'm working on, we have a requirement for adjacent nested lists. I have been digging to find where or how to update this since I could potentially contribute this feature.

I'm filing this issue to express interest in this feature and to see if anyone can point me in the right direction. From what I have seen so far code changes may need to happen outside of the scope of the AdjacentListsSupport plugin for this to work.

Using the manual tests, you can test the AdjacentListsSupport plugin using documentlist-separator.html. Example code to test:

<ol>
  <li>One
    <ol>
      <li>1.1</li>
    </ol>
    <ol>
      <li>1.2</li>
    </ol>
  </li>
  <li>Two</li>
</ol>

Current result:

<ol>
  <li>One
    <ol>
      <li>1.1</li>
    </ol>
  </li>
</ol>
<ol>
  <li>1.2</li>
</ol>
<ol>
  <li>Two</li>
</ol>

---

If you'd like to see this feature implemented, add a 👍 reaction to this post.

[star-szr]

I've worked around this for now using a modified version of AdjacentListsSupport that uses a listSeparator attribute on a paragraph model rather than defining a separate listSeparator model. I think it's probably a hack and not worthy of a PR, but at this point I have spent too much time on it and I'm not sure what a more thorough solution would look like.

[wimleers]

Drupal issue: https://www.drupal.org/project/drupal/issues/3402481

[Witoso]

Let me change it to a bug.