BUG: Horizontal merging of table cells doesn't work when header column is used

[marco-wildermuth]

Description

When using tables in the rich text editor in the backend it is not possible to merge cells horizontally if there is a header column.

Steps to Reproduce

Tested on https://demo.neos.io/, but same result in our project.

1. Insert a table into a rich text field, maybe 3x3
2. Select a cell in the first column and mark the column as header column
3. Select a cell in the second column and use "Merge cell right" - the cells do not merge
4. Unmark the first column as header column
5. Select a cell in the second column and use "Merge cell right" - the cells merge
6. The first column can now be marked as header column

When using the CKEditor demo https://ckeditor.com/ckeditor-5/demo/feature-rich/ this can't be reproduced.

Expected behavior

The cells should be able to be merged horizontally even if the table has a header column.

Actual behavior

Affected Versions

Neos: 8.3

UI: 8.3.0-8.3.6

[mhsdesign]

This might be caused by the discrepancy in the ckeditor versions. Neos 8.3 currently uses CKeditor 16, while - as not using semver - have a quick release schedule and probably are somewhere at release 20 or even 30.

Maybe this problem will be fixed by updating to a higher ckeditor version, which is an open task, but also a dilemma as it might break plugins and additionally will require lots of dedication. See #2836

On the otherhand it might be that we just use ckeditor wrongly and version 16 would work but not our adaption. To check that i would look if there are any console errors and if a ck16 plain install works