Markers removal

[pjasiun]

Markers removal is now implemented similarly to the attribute removal: based on marker name/descriptor converters decide what kind of elements should be removed and unwraps similar elements.

The problem is that markers do not work like attributes. For attributes it is not important which instance of an element should be removed: if <b> suppose to be removed from foo it does not matter which <b> is removed as long as foo is unbolded. If a comment or users selection should be removed it is very important to remove the proper element.

Also, markers values change based on data stored outside the document model much more often than attributes do. In fact, this is what markers were created for: to show some external data on the document (like comments or users selection). The problem is that it means that markers descriptors can change asynchronously. For instance, if a comment is selected marker gets a different color. Because of it, the marker to remove is not the same as the marker which was added. It leads to many bugs and hacks.

The third problem is the current way of conversation, introduced together with differ. Because now conversion is delayed it is very hard to find the range of the makers in the view, when model changed.

This is why I think that markers should be removed in a different way. When elements are added to the view they should be stored and exactly these elements should be removed. The only information sent to the converters should be the name of the marker to be removed because the descriptor and range can be invalid at the moment of removal.

After this short introduction, let's move to details.

There are 2 mechanisms to be changed to implement it.

First is view writer. View writer is designed to work with attributes. It splits attribute elements with no information about their connection. The only way to unwrap is by giving range and element template - all similar elements are removed, what is fine for attributes but not for markers. Whenever view writer split view element it should store information about this elements parts. Then it should be possible to unwrap certain element (and all its parts) using a reference to the element instance, not by using range and template. The proper set of view writer methods should be provided.

Second, is the way how converters work. Markers converters should store information what element has been added to the view and what element should be removed. Then it should be enough to say which marker should be removed, based on its name. Range and descriptor should not be needed when a marker is removed.

[scofalik]

Short note: maybe we could remove HighlightAttributeElement. As far as we remember, it was introduced because of problems with unwrapping attribute elements.

[pjasiun]

See https://github.com/ckeditor/ckeditor5-engine/issues/1303. HighlightAttributeElement need to be removed anyway, because after recent change in the view, subclassing view elements is not allowed.