I actually used this pattern as well and I really like it. I store another property called `mark_idx`. When `cursor_idx` and `mark_idx` are the same then it's j

I actually used this pattern as well and I really like it. I store another property called `mark_idx`. When `cursor_idx` and `mark_idx` are the same then it's just a cursor. When they differ, the range between them is the selection.