While writing code in a new Pen, the editor is by default split up into three panels: HTML, CSS, and JS, each of which take up 1/3 of the width or height of the screen, depending on the layout. Often times this is enough space to write comfortable, sometimes it isn’t.
Take for instance writing CSS gradients out. The default width can be too narrow to write and see the code comfortably.
You can expand any of the three editors by clicking the expand icon in the title bar of the editor (four arrows pointing outwards).
You can contract the editor by pressing the Esc key on the keyboard or clicking the icon again.