In the MAC OS, when choosing “New Editor Window (PIE)” it is expected behavior to see the top border and the screen to occupy its own Window. However there is erratic behavior upon returning to the editor after entering the Console Command: “r.SetRes 1920x1080f.” This has been reported in JIRA [UE-21914].
If you want to enter fullscreen you can use the same procedures listed for Windows: press f11 while Playing in the editor viewport or in a Standalone Game, type the console command: fullscreen (or “r.SetRes -any resolution- f”)