The NotebookTextModel is an important backbone of the notebooks model and therefore we should cleanup some debt that has accumulated:

• NotebookTextModel#selections is a view/editor property and not a model property
• the this._onDidModelChangeProxy-event in combination with emitToExtHost. There should be no difference is model changes that go to the extension host and other subscribers/consumers. All events should be equal and mainThreadNotebook is just one of many consumers which special role is to manage a "copy" in the extension host
• some duplication in how cells are inserted, removed, and edited. We should make applyEdit the one-stop-shop.
• a notebook text model knows if its dirty and emit corresponding events. One could argue that dirty-state manage is the business of NotebookEditorModel which is already doing saving etc.
• every change of the version id must be forwarded to the extension host (see Using applyEdit with replaceCellMetadata or replaceCellOutput fails with no output #105624 (comment))