Weird case with attribute removal in model post-fixer

[jodator]

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

🐞 Bug report

💻 Version of CKEditor

latest master

📋 Steps to reproduce

1. Register attribute post-fixer:

editor.model.document.registerPostFixer( writer => {
	const changes = editor.model.document.differ.getChanges();

	let wasChanged = false;

	for ( const change of changes ) {
		if ( change.type == 'insert' || change.type == 'remove' ) {
			const textNode = change.position.textNode;

			if ( change.name == '$text' && textNode && textNode.hasAttribute( 'bold' ) ) {

				writer.removeAttribute( 'bold', textNode );
				wasChanged = true;
			}
		}
	}

	return wasChanged;
} );

2. bold text ie bold
3. delete some char, ie l (forward or backward delete)
4. the model has correct state <paragraph><$text bold="true">bo[]d</$text></paragraph>
5. the view has incorrect state: <p>bo[]l</p>
6. execute undo

✅ Expected result

The text after deletion is bod in the view.
The text after undo id bold in the view.
The text after undo has bold attribute in the model.

❎ Actual result

The text after deletion in the editing area is bol.
The text after undo id boll in the view.
The text after undo doesn't have bold attribute.

📃 Other details that might be useful

Screen cast:

[jodator]

ps.: The same happens on FF and Chrome.

[scofalik]

I confirm the bug.

Quick observation: when removing, always the last letter is removed, unless the first letter is removed. When typing, the typing itself is fine but undo fails.

[scofalik]

This is a bug in Differ. A quite serious one, it is funny that it didn't come up earlier but I guess we never had a scenario where a word changed an attribute and also got something removed/added.

[scofalik]

Okaaay, so ckeditor/ckeditor5-engine#1705 this fixes the issue with Differ. Attribute change was incorrectly stored if there was a remove change with intersecting range.

However, this does not fix not-bolding on undo. In fact, everything works correctly. The bold is applied, but since also there is a letter inserted (or removed) during the undo, the bold is immediately taken off.

So the post-fixer like this for mentions is not correct.