Heads up! This blog post hasn't been updated in over 2 years. CodePen is an ever changing place, so if this post references features, you're probably better off checking the docs. Get in touch with support if you have further questions.
A few of you have asked for this feature. It occurred to me that not only is it useful, it would also be pretty easy to implement since we were already doing layout math stuff on resizes. Might as well just display the useful numbers on the screen!
Above is how it works on the left and right layouts. When the editors are in the top position, we still display it in the resizer bar, it’s just centered in the middle: