src/vs/workbench/api/browser/mainThreadNotebookDocumentsAndEditors.ts/MainThreadNotebooksAndEditors/_handleEditorAdd
class MainThreadNotebooksAndEditors:	private _handleEditorAdd(editor: INotebookEditor): void {
		this._editorListeners.set(editor.getId(), combinedDisposable(
			editor.onDidChangeModel(() => this._updateState()),
			editor.onDidFocusWidget(() => this._updateState(editor)),
		));
		this._updateState();
	}