Steps to reproduce:

1. Click the link to a (preferrably very large) script - I've used http://download-mirror.savannah.gnu.org/releases/linterna-magica/linternamagica-v0.0.12.user.js.html for testing. For repeated tests, the cache has be cleared before clicking the link again.
2. Immediately after the install dialog shows up, click "Show Script Source"

Expected result: Script source shows in new tab
Actual result: RemoteScript#showSource throws Script is not loaded!

When removing the check for _progress[0] < 1, everything seems to work as expected.