Merging down double-row header cell results in broken table

[f1ames]

Is this a bug report or feature request? (choose one)

🐞 Bug report

💻 Version of CKEditor

v10.1.0

📋 Steps to reproduce

1. Go to https://docs.ckeditor.com/ckeditor5/latest/features/table.html.
2. Place selection in the top-left cell.
3. Open merging/splitting panel and apply Merge cell down.

✅ Expected result

Cell is correctly merged.

❎ Actual result

Cell is not merged and the cell below disappears.

📃 Other details that might be useful

Tested on Chrome 67.0.3396.99 .

Looks like this is the issue with thead, works fine if there is only tbody. Also if thead is a single row, Merge cell down option is disabled.

[jodator]

Cell is correctly merged.

The expected result is that the merge shouldn't be possible - the mege would cross <thead> & <tbody> boundary which isn't allowed - now this happens:

<th rowspan="3"></th>

So this table cell is "properly" expanded (from rowspan="2"), the cell is merged with one below but the browser (correctly) doesn't render this and we have a collision.

Probably an easy/small fix though.

ps.: The merge up is properly grayed-out.