Steps to reproduce the problem:
1. Open "primary" site in a Progressive Web App (PWA) from device home screen
2. Click link to go to different ("external") site
3. External site opens in default OS browser (Chrome)
4. External site redirects browser back to primary site. (In my use case it redirects back with an OAuth token.)
5. Primary site appears in Chrome browser, NOT in PWA. User has no way to return to the PWA.

What is the expected behavior?
When the "external" site redirects back to the "primary" site, the primary site should reopen in the PWA, instead of in the browser.

Or another option: the browser should show a "return to PWA" button—when clicked it would open the page in the PWA.

What went wrong?
The user has no way to return to the PWA after exiting to an external site.

Did this work before? No 

Does this work in other browsers? N/A

Chrome version: 60.0.3112.107  Channel: stable
OS Version: 6.0.1
Flash Version: 

My use-case for this is OAuth. I'm working on an PWA that can be added to the device home screen. When the user opens the app, they are prompted to log in via Goodreads (a book website) via OAuth 1. The PWA opens a Chrome browser (non PWA) window at Goodreads.com where the user can log in. After login, Goodreads redirects back to the primary site with a token. But the user cannot return to the PWA app—they are stuck in the browser.