Node edit protection does not protect against navigating away from preview page

If instead of clicking Save, the user clicks Preview, and then navigates away from that, the work is (potentially) lost.

Steps:
1. Log in
2. Add content
3. Choose a content type, e.g., Basic Page
4. Type in all required fields
5. Click Preview
6. Click Home button

Actual results: Navigates away from page without warning.
Expected results: Gives navigation warning. That is, preview is assumed dirty.

(In this instance, I was able to use the back button and retained my work, but had step 6 been Click Log out, the work would have been lost.)