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I once saw an @CodePen tweet that was like uhhhhh NOT LOVING the UI changes to the editor. Turns out, they were getting the Mobile Editor, which is a completely different UI than our normal Editor. Admittedly, the Mobile Editor doesn't scale up to large screens with the best UI. It's not meant to.
The reason you might get the Mobile Editor instead of the normal Editor is because you are loading CodePen for the first time with your browser window very narrow. Maybe you have DevTools open and docked on the side. We detect that width and save it in a cookie so we can use that information server side (to serve the correct Editor).
We used to tell people "Oh, hey, clear your cookies. Or specifically the cookie on codepen.io named
screen_width". But that's kinda lame. Now you'll get this: