Markers should be properly downcasted and upcasted on bogus paragraphs

[niegowski]

Suggested merge commit message (convention)

Fix (engine, list, table): Markers should not be lost on list items and in table cells in the data pipeline. Closes #13285.

---

Additional information

[Mgsy]

Not a big deal, but I can't reproduce it on release:

Steps to reproduce

1. Go to http://localhost:8125/ckeditor5/external/collaboration-features/packages/ckeditor5-collaboration/tests/manual/all-features-rtc-document-list.html.
2. Insert an empty table and put the selection inside an empty cell.
3. Turn on Track Changes.
4. Select Heading 1 from the headings dropdown to add a formatting suggestion.
5. Call editor.data.set( editor.getData(), { suppressErrorInCollaboration: true } ).
6. Reject the suggestion.
7. Try to create an ordered list using autoformatting in the cell that had a formatting suggestion.

Actual result

It's impossible to insert a list using autoformatting.

Notes

Just noticed it could be reproduced only with editor.data.set( editor.getData(), { suppressErrorInCollaboration: true } ). Reconnecting to the document doesn't trigger this issue, so I guess we can ignore it.

[scofalik]

AFAICS, this happens because there is a bogus nbsp; added to the table cell.

editor.getData() returns this:

<figure class="table"><table><tbody><tr><td><p data-suggestion-start-before="formatBlock:698dn3bun9mf:e321952b3e500000ea901c56954d7ddcd:e168fe479815a10c23f0c08a44996fe42"><suggestion-end name="formatBlock:698dn3bun9mf:e321952b3e500000ea901c56954d7ddcd:e168fe479815a10c23f0c08a44996fe42"></suggestion-end>&nbsp;</p></td></tr></tbody></table></figure>

@niegowski, can we do anything relatively simple to avoid these spaces?

[niegowski]

AFAICS, this happens because there is a bogus nbsp; added to the table cell.

editor.getData() returns this:

<figure class="table"><table><tbody><tr><td><p data-suggestion-start-before="formatBlock:698dn3bun9mf:e321952b3e500000ea901c56954d7ddcd:e168fe479815a10c23f0c08a44996fe42"><suggestion-end name="formatBlock:698dn3bun9mf:e321952b3e500000ea901c56954d7ddcd:e168fe479815a10c23f0c08a44996fe42"></suggestion-end>&nbsp;</p></td></tr></tbody></table></figure>

@niegowski, can we do anything relatively simple to avoid these spaces?

This is a block filler that was not stripped because of another element in the same block (there is a marker-end element) and DomConverter is not able to strip this block filler because it does not know whether the marker element is a marker or any other element. We might teach DomConverter what element names are describing markers (by registering element names while registering a marker converter) to ignore such elements while checking block filler but I'm not sure if we could do it in such a short time window. This looks like sth out of the scope of this ticket and I would suggest extracting this to a separate issue. WDYT @scofalik

[scofalik]

I thought so.

I briefly look whether this won't mess up with revision history, but it seems it is fine.