User blog comment:Kirkburn/Technical Update: August 5, 2015/@comment-25648422-20150811155635/@comment-126761-20150811193252

It sounds like you're talking about widescreen mode in the classic editor? That's actually been around for many years, and is linked to a user preference ("Widen the Source mode edit box to fill the entire screen"). You can freely open and close it on most pages (the main exception being the main page).