The essential idea of a cursor can be implemented in about 10 lines, and because I actually looked through the source code of mobservable, I understand what they are essentially doing and how it requires a couple hundred lines. Since we're talking ideas here and not implementations, measuring artifacts is not helpful.
1 - https://github.com/dustingetz/react-cursor/blob/master/dist/... 2 - https://github.com/mweststrate/mobservable/blob/master/dist/...