src/vs/workbench/contrib/notebook/browser/viewModel/cellEdit.ts/JoinCellEdit/undo
class JoinCellEdit:	async undo(): Promise<void> {
		if (!this.editingDelegate.insertCell || !this.editingDelegate.createCellViewModel) {
			throw new Error('Notebook Insert Cell not implemented for Undo/Redo');
		}

		await this.cell.resolveTextModel();

		this.cell.textModel?.applyEdits([
			{ range: this.inverseRange, text: '' }
		]);

		this.cell.setSelections(this.selections);

		const cell = this.editingDelegate.createCellViewModel(this._deletedRawCell);
		if (this.direction === 'above') {
			this.editingDelegate.insertCell(this.index, this._deletedRawCell, { kind: SelectionStateType.Handle, primary: cell.handle, selections: [cell.handle] });
			cell.focusMode = CellFocusMode.Editor;
		} else {
			this.editingDelegate.insertCell(this.index, cell.model, { kind: SelectionStateType.Handle, primary: this.cell.handle, selections: [this.cell.handle] });
			this.cell.focusMode = CellFocusMode.Editor;
		}
	}