I think this is inspired by the fullscreen mode in Wordpress. That's a good thing. Now I wish other places would incorporate this. I'd love a zen mode in Gmail, for instance.
You mean instead of the "let's take a step backward and give you a tiny window in the bottom right corner of your screen?"
That would be nice.
For gmail the new compose allows full screen mode which is very distraction free and clean UI. http://i.imgur.com/jrqOfiQ.png
I wasn't even aware this was a thing. Google's been busy updating their older stuff as of late- the new image search took me by surprise just earlier today.
It's an editing mode initially re-popularised by an OS X app called WriteRoom.