Unfullscreen button is not discoverable for non-geeks: no tooltip and no label
With Epiphany (technology preview), the only difference between the regular headerbar and the fullscreened headerbar is the presence of this un-fullscreen button:
However, it is 1) not very noticeable 2) not necessarily obvious to non-geeks 3) the only button in the toolbar/headerbar that does not have a tooltip.
For accesibility and discoverability, it should at least have a tooltip property set, but I would go even as far as to suggest that it should have a full user-readable label so that it is bigger and "obvious", so users cannot get stuck into that UI mode.