Fixing validation, highlighting and authoring state

[yuricus]

No description provided.

[yuricus]

LGTM

[AlexeyMz]

If you use Set<ElementIri> instead of an array, you'll be able to perform this test in two calls:

elementIris.has(linkModel.link.sourceId) || elementIris.has(link.targetId);

[AlexeyMz]

I recommend testing this directly (without isNew variable):

if (!(event.type === AuthoringKind.ChangeLink && !event.before)) { return true; }

This way you won't need a cast later: (event as LinkChange)

[AlexeyMz]

It should be easier to replace this whole if-statement with something like this:

this.discardChange(event);
this.model.removeElement(item.id);

This way you don't even need to have deleteNewLinksReleatedToElements() method at all.

[AlexeyMz]

I think this block could be simplified by reusing discardChange() logic a little bit more:

// remove new connected links
const linksToRemove = new Set<string>();
for (const element of elements) {
    for (const link of element.links) {
        if (link.data && AuthoringState.isNewLink(state, link.data)) {
            linksToRemove.add(link.id);
        }
    }
}
// delete separately to prevent modification while iterating
linksToRemove.forEach(linkId => this.model.removeLink(linkId));

if (event) {
    this.discardChange(event);
}

[AlexeyMz]

What's so dangerous about these?