Thanks @mccraigmccraig. Not sure I understand the issue completely, but it sounds difficult to solve.

@callum chains of Deferreds are all callbacks under the hood... the callbacks are executed when the Deferred has success! or error! called on it - but who knows when or which Thread that will happen on. this is also worth a read for some more info - https://github.com/ztellman/manifold/blob/master/docs/execution.md