Currently it's ctrl+D.
Tip: You can also use F1 to check available shortcuts/actions in "command palette" popup window. Took me an hour or so to find that command palette shortcut (as a new user)...

If anyone from jsfiddle is reading this... Please update your docs page...
Why is it not mentioned anywhere in the docs? Why is there no cheat sheet with keyboard shortcuts in the docs or even better in the editor itself?
Your docs page is very basic, poorly maintained and contains barely any useful information for such widely used tool in coding community...