If you look closely at https://github.com/mozilla/popcorn.webmaker.org/blob/master/public/src/timeline/trackhandles.js#L34 you will notice it achieves this by listening to an event called "sortstarted" which is dispatched at https://github.com/mozilla/popcorn.webmaker.org/blob/master/public/src/util/dragndrop.js#L1101. You could apply a similar concept/idea and listen for "sortstopped" which is dispatch at https://github.com/mozilla/popcorn.webmaker.org/blob/master/public/src/util/dragndrop.js#L1116