The auto-save behavior in Composer is triggered in a purely event-based manner and will only auto save in the following cases:
Opening the story after creating it will create a revision
The story has unsaved changes, and user switches to a different browser tab
User clicks on revision (view revisions)
The story is taken over by someone else
The lock expires due to idle
However, our users expect the auto-save feature to be a full, continuous save (think Google Docs).
This would help greatly in helping the auto-save behavior be more congruent with user expectations and can help prevent accidental data loss.