src/vs/workbench/api/browser/mainThreadNotebookDocumentsAndEditors.ts/MainThreadNotebooksAndEditors/_onDelta
class MainThreadNotebooksAndEditors:	private _onDelta(delta: INotebookAndEditorDelta): void {
		if (MainThreadNotebooksAndEditors._isDeltaEmpty(delta)) {
			return;
		}

		const dto: INotebookDocumentsAndEditorsDelta = {
			removedDocuments: delta.removedDocuments,
			removedEditors: delta.removedEditors,
			newActiveEditor: delta.newActiveEditor,
			visibleEditors: delta.visibleEditors,
			addedDocuments: delta.addedDocuments.map(MainThreadNotebooksAndEditors._asModelAddData),
			addedEditors: delta.addedEditors.map(this._asEditorAddData, this),
		};

		// send to extension FIRST
		this._proxy.$acceptDocumentAndEditorsDelta(new SerializableObjectWithBuffers(dto));

		// handle internally
		this._mainThreadEditors.handleEditorsRemoved(delta.removedEditors);
		this._mainThreadNotebooks.handleNotebooksRemoved(delta.removedDocuments);
		this._mainThreadNotebooks.handleNotebooksAdded(delta.addedDocuments);
		this._mainThreadEditors.handleEditorsAdded(delta.addedEditors);
	}