Roundup already supports detecting changes to an issue with:

 * HTML pages includes a hidden @lastactivity form input
 * REST API updates require an etag that must match the servers current state for the object
 * XML-RPC may not have any protection see issue2551304

however all (well except xml-rpc) detect changes only on submission.

If a user has a long running open issue and a change is done by somebody else,
it would be nice to notify the user and optionally apply that change and update
the @lastactivity/etag to bring the data in sync with the server. 

One possible way to do this is to use a long poll from a javascript worker.
One example of a long poll using python:

   https://blog.fanout.io/2016/11/21/moving-from-polling-to-long-polling/

Note this doesn't replace the existing collision detection mechanisms, it just makes
it nicer for the user.