We should be able to add a unit test which tests the use-case for the intended fix. I added a comment in the bug, but essentially I think we would need to call the load method twice in a row, and ensure the second callback fires after the first (when the resources are still loading during the second call).

The tests are currently here: https://github.com/mozilla-b2g/gaia/blob/master/apps/gallery/test/unit/lazy_loader_test.js