Editor crashes after merging paragraph to blockquote in table cell

[Mgsy]

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

🐞 Bug report

💻 Version of CKEditor

Latest master.

📋 Steps to reproduce

1. Go to https://ckeditor5.github.io/docs/nightly/ckeditor5/latest/examples/builds/document-editor.html.
2. Insert 1x2 table.
3. In the first cell insert a blockquote.
4. In the second cell type something.
5. Merge both cells.
6. Put the caret at the beginning of the paragraph in the merged cell.
7. Press Backspace.

✅ Expected result

The editor doesn't crash.

❎ Actual result

The editor crashes.

📃 Other details that might be useful

Error

moveoperation.js:171 Uncaught CKEditorError: move-operation-node-into-itself: Trying to move a range of nodes into one of nodes from that range. Read more: https://ckeditor.com/docs/ckeditor5/latest/framework/guides/support/error-codes.html#error-move-operation-node-into-itself
    at MoveOperation._validate (webpack://Letters/./packages/@ckeditor/ckeditor5-engine/src/model/operation/moveoperation.js?:171:12)
    at Model.on (webpack://Letters/./packages/@ckeditor/ckeditor5-engine/src/model/model.js?:99:14)
    at Model.fire (webpack://Letters/./packages/@ckeditor/ckeditor5-utils/src/emittermixin.js?:203:29)
    at Model.(anonymous function) [as applyOperation] (webpack://Letters/./packages/@ckeditor/ckeditor5-utils/src/observablemixin.js?:257:16)
    at Writer.move (webpack://Letters/./packages/@ckeditor/ckeditor5-engine/src/model/writer.js?:507:14)
    at Writer.insert (webpack://Letters/./packages/@ckeditor/ckeditor5-engine/src/model/writer.js?:193:10)
    at mergeBranches (webpack://Letters/./packages/@ckeditor/ckeditor5-engine/src/model/utils/deletecontent.js?:153:10)
    at mergeBranches (webpack://Letters/./packages/@ckeditor/ckeditor5-engine/src/model/utils/deletecontent.js?:175:2)
    at model.change.writer (webpack://Letters/./packages/@ckeditor/ckeditor5-engine/src/model/utils/deletecontent.js?:88:4)
    at Model.change (webpack://Letters/./packages/@ckeditor/ckeditor5-engine/src/model/model.js?:173:11)

GIF

[Reinmar]

OK, some weird thing is happening here:

1. 

2. 

3. 

It looks like we're not handling this Backspace but leave it to the browser (which is the one creating that weird <span>). That should not happen – we should be handling that Backspace...

[jodator]

Taking this as already on it: https://github.com/cksource/cloud-features/issues/2205.

@pomek found out that this case does not happen without table and it isn't associated with table post fixers.

[pomek]

Fix for the issue: https://github.com/ckeditor/ckeditor5-engine/blob/99def977bebea4ae342df535cd133d8f63df5825/src/model/utils/deletecontent.js#L142-L148

Here we should check whether endParent and startPos.parent aren't the same elements.

if ( endParent == startPos.parent ) {
	return;
}

if ( !endPos.isEqual( startPos ) ) {
	// In this case, before we merge, we need to move `endParent` to the `startPos`:
	// <a><b>x[]</b></a><c><d>{}y</d></c>
	// becomes:
	// <a><b>x</b>[]<d>y</d></a><c>{}</c>

	writer.insert( endParent, startPos );
}