Adjust (expand) the margins of the text editor

A simple one this; how do I expand the margins of the text editor so that the text takes up the width of the editor window?

The ruler bar seems locked at 0, but that leaves 3.5 inches of white screen space that I’d rather was taken up with text. Plus, when I close the inspector, the text jumps to a position 3/5ths of the way across the screen and not the centre.

I just want the old view back. :cry:

Tweak this window as needed:

A thousand thank-yous

Welcome.