dom.EmitterMixin#stopListening() doesn't clear the proxy emitter

[Reinmar]

After changes in ckeditor/ckeditor5-utils#190 calling stopListening() correctly removes all listeners, even those added via on().

However, it seems that dom.EmitterMixin#stopListening() doesn't clean up things in the ProxyEmitter because it calls detach( undefined ). It should be investigated.

[jodator]

Just initial thoughts on this as I didn't dig into it much:

I think that it will work the same way as EmitterMixin.stopListening().

So

domEmitter.stopListening( domNode );

will call stopListening on all defined events (thus detach with event) and then final detach( undefined ):

https://github.com/ckeditor/ckeditor5-utils/blob/f1a92c1e4e4a5c37933a09ef48a416bdf334db12/src/emittermixin.js#L219-L225

Also similary this happens with domEmitter.stopListening() it does detach all defined events.

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What is unclear at first spot is maybe the fact that stopListening() method is very recursive and to analyze that method behavior it's just easier to console.log() stuff rather then read through it.

Maybe it would be better to split this method somehow to smaller - strictly defined methods - without recurrence? 🤔

[Reinmar]

I certainly don't like the EmitterMixin's code now. It grew to a very ugly blob of hardly readable code. I tried to analyse it but I failed. So I agree that it should be better.

But the first thing we need is having more tests. Please write extensive tests for removing listeners. That will tell us if we should investigate it any further because it doesn't make sense to touch now code which works. At the same time, new tests may be helpful when we'll be rewriting this code one day.

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