> This is a fairly specific itch. I use one browser window tiled directly in front of me, usually with its sidebar open. When a site deliberately centers a narrow layout, I want it at the dead center of that window, not the space left over beside the sidebar.

I still don't understand the need for this. I mean, it's hn, so "I wanted to" is a perfectly acceptable reason, but when you resize and move the browser window, you don't expect the content to remain where it was. The browser window is not a literal window that shows you what's behind your screen. If the sidebar should behave like an overlay, then that's something that the browser should manage, not each website. From the blogpost I can see that this preference is highly subjective, so it being a browser-wide setting makes much more sense.