M49 beta
Mac 10.11.x

Popup windows (those without a tab strip or full toolbar and only an omnibox) now have the option to be turned back into real tabs with Window > Show as tab. However, this isn't discoverable because it only appears for popup windows. When a normal window has focus, it's hidden.

It's not good UI to hide things like this, as it makes them virtually undiscoverable. We should always show this item and disable it instead of hiding.