Back in April, after Google pulled the option of having tabs and apps merged in Recents, it became apparent that the Chrome team was trying to work out how to move forward. The team placed a new flag in Chrome Dev 52, #tab-management-experiment-type, with five 'flavors' of tab management: Asine, Basil, Chive, Dill, and Elderberry. In Chrome Dev 54, this has now come to a head, with Elderberry being the only option remaining.
A lot of apps have built-in link handling with Chrome Custom Tabs - we're all so used to this now that we hardly think about it. A few apps do not have this, however, Gmail being possibly the biggest one to miss out on the party. With Chrome Dev (54) set as the default browser, instead of opening the entire Chrome app, a tab will open that looks and feels like a Custom Tab, but in fact it's just a normal Chrome Dev tab, packaged slightly differently. It's kind of a halfway house between a Chrome Custom Tab and the full Chrome browser. These tabs get their own entries into Recents (at least they do in Gmail; Facebook Messenger does not do this), and can be opened in the full browser by tapping on the overflow. This is the Elderberry flavor, and it looks set to become the default option once Chrome Dev 54 becomes Chrome 54.
Left: an Elderberry tab. Right: a tab's entry in Recents.
Chrome 54 is set to arrive in mid-October, barring any bugs or unforseen circumstances. In the meantime, you can check out the Elderberry tab management flavor by downloading Chrome Dev on Google Play or APK Mirror and setting it as your default browser in System Settings.