The editor on the Mac has nice preferences that allow you to define the size of the margins. Right now, I have the Margin setting set to the maximum size. I tend to like to write in full screen with all sidebars hidden (its as close as we can get to a distraction free mode). When I open the sidebars, my content area shrinks, not the margins.
However, I feel that I need the opposite. I should define the maximum width of the content area. This way when I open or close sidebars, the content width remains the same and the margins are what adjust.
Obviously there needs to be a minimum margin so that if the window is too small the content does not go all of the way to the edge. The content would also reduce its width as well if there is not enough space. Sort of like a responsive website.