Wrong list style in undo stack after merging two different list styles

[mlewand]

📝 Provide detailed reproduction steps (if any)

1. Open lists feature demo for the upcoming v21 release (not not published yet, so production link won't work).
2. Focus the editor under the "List styles" section.
3. Select entire first list.
4. Change style to upper-roman.
5. Remove the entire "How to prepare the coffee" paragraph, so that the list get combined.
6. Undo.

Full screencast:

✔️ Expected result

Both lists preserve their initial styles (the first keeps upper Roman, the second keeps numeric with leading 0)

❌ Actual result

You're unable to restore initial list type of the second list.

📃 Other details

• Browser: Chrome
• OS: Windows 10
• CKEditor version: v21

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[pomek]

The model after merging lists:

The model after applying the undo:

While pressing undo, the model has not changed and that's why the view is invalid.

[pomek]

The issue was fixed (accidentally) while we merged #7902.

Before merging #7902, lists were merged in the view but the model state was invalid. When applying the undo command, the model hasn't changed and the view wasn't rendered.

After merging #7902, the model was updated (change the listStyle attribute), the view could render those changes. When applying the undo command, the model was reverted and the view display the proper state of the model.