Inserting node into some element should remove it from its parent

[Reinmar]

I wonder how many times we do this:

someElement.appendChild( someTree.getChild( 0 ) );

Every time we move a node we may be doing exactly this. And this means that such a node ends up in being someTree's and someElement's child.

Fortunately, in the view we bubble events using the parent chain so the events bubble to the new tree. But it may lead to issues anyway, especially in the tests.

E.g. I've just seen a code which did this:

const el = somElement.getChild( 0 );

new ViewElement( 'foo', null, [ el ] );

Which has exactly the same meaning as the move operation above. I don't want to be the first person who will spend a day debugging an issue caused by a node being in two places so I think we should prevent it.

[Reinmar]

Actually, it'll be easy to check how often we do such operations by checking the parent of node passed to insertChildren() :D

[scofalik]

I think it won't be as many as you think because those bugs often show up pretty fast. But I agree for 100% that this should be changed. I myself did this kind of error twice already.

[pjasiun]

I agree that appendChild should remove them from the previous parent. In the model, https://github.com/ckeditor/ckeditor5-engine/issues/858 will (should) solver it. However, we should solve it in the view too.

[Reinmar]

This problem caused https://github.com/ckeditor/ckeditor5-engine/issues/1139.

[Reinmar]

Fixed with https://github.com/ckeditor/ckeditor5-engine/issues/1139.