User blog comment:DaNASCAT/Technical Update: November 23, 2015/@comment-1398253-20151124034524

From the Nov 18 update:
 * A rogue elephant recently sat on the Visual/Source tabs in the classic editor, causing them to get a little squished. They are receiving medical attention, and should be back to their normal selves in the next site update.

Glad to see this fixed. Let's not see those elephants again, eh? :)