Error when merging paragraphs and undo

[FilipTokarski]

📝 Provide detailed reproduction steps (if any)

1. Open any editor
2. Create two paragraphs with text content
3. Merge them by putting cursor at the beginning of the second one and pressing backspace
4. Delete the whole new paragraph
5. undo 4 times

✔️ Expected result

Undo works without errors. Content looks like in step 2

❌ Actual result

Console error:

Uncaught CKEditorError: merge-operation-target-position-invalid
Read more: https://ckeditor.com/docs/ckeditor5/latest/framework/guides/support/error-codes.html#error-merge-operation-target-position-invalid
    at MergeOperation._validate (http://localhost:8125/ckeditor5/tests/manual/all-features.js:61573:10)
    at Model.on.priority (http://localhost:8125/ckeditor5/tests/manual/all-features.js:59082:14)
    at Model.fire (http://localhost:8125/ckeditor5/tests/manual/all-features.js:156788:30)
    at Model.<computed> [as applyOperation] (http://localhost:8125/ckeditor5/tests/manual/all-features.js:159187:16)
    at UndoCommand._undo (http://localhost:8125/ckeditor5/tests/manual/all-features.js:150692:11)
    at Object.callback (http://localhost:8125/ckeditor5/tests/manual/all-features.js:151010:9)
    at Model._runPendingChanges (http://localhost:8125/ckeditor5/tests/manual/all-features.js:59821:58)
    at Model.enqueueChange (http://localhost:8125/ckeditor5/tests/manual/all-features.js:59227:10)
    at UndoCommand.execute (http://localhost:8125/ckeditor5/tests/manual/all-features.js:151009:21)
    at UndoCommand.<anonymous> (http://localhost:8125/ckeditor5/tests/manual/all-features.js:159183:32)

Screencast

0_undo1.mp4

📃 Other details

• Browser: any
• First affected CKEditor version: very old, even on version 15

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[scofalik]

Could you check if it works on i/8870?

[FilipTokarski]

Actually, I found it when testing i/8870