It appears as if rank is stored as a float. The rank can quickly get to be a very, very small number. It seems at some point, float precision might be an issue and/or length of the rank key. Have you run into this and/or do you have solutions?

Brandon Keepers wrote about trying this idea in Ruby http://opensoul.org/blog/archives/2011/09/06/sortable-lists-... and ran into a float precision issue (see comment).

Maybe I’m using outdated terminology, but why is this called a “rendering engine?” When I think of rendering engines, I think of things that generate pixels.

Back in an earlier version of Meteor I actually implemented drag-sortable and drag-drop lists with jQuery UI and Reactive data sources and it wasn't very difficult. I can't post the source - but there is a simplified demo here: http://grass-freight.meteor.com/