src/vs/workbench/api/common/extHostNotebookConcatDocument.ts/ExtHostNotebookConcatDocument/offsetAt
class ExtHostNotebookConcatDocument:	offsetAt(position: vscode.Position): number {
		const idx = this._cellLines.getIndexOf(position.line);
		const offset1 = this._cellLengths.getPrefixSum(idx.index - 1);
		const offset2 = this._cells[idx.index].document.offsetAt(position.with(idx.remainder));
		return offset1 + offset2;
	}