Admin screen no longer full width? Orange text?

The whole layout of the Admin area seems to have changed suddenly. The Screen Options tabs and other elements have all moved. The working area on different admin pages (Post, jetpack etc) is no longer full-width, it’s boxed. And this strange orange text highlight has spread across the pages like a fungus.

I am the adminstrator on the site and I am positive it isn’t something I did.

Was it you, WordPress?!

And how do I go back (to what worked)?