The Stack Exchange website is a annoying to view at anything but the Stack Exchange Official Window Width. (Which is about to change.) Use a narrow window, and you get truncated text. Use a wide window and you get a lot of blank space. (Except for a few moderator tools which are exclusively available from the otherwise blank left margin and are not visible at all unless you make your window wider than the SEOWW!)
Please let us get a comfortable experience at our choice of window width, chosen to suit our monitor setup, eyesight and window placement habits. Browser window size affects every site you use, so "fixing" it for SE can just break something else. The window width is a user choice, not a site choice.
Can SO have a fluid vs. fixed-width layout? was declined, apparently on the basis that if you have a wider window than the SEOWW, it's not too bad. But for those of us who have a preferred window width narrower than the SEOWW, it's much worse: there's some stuff we have to keep scrolling horizontally for, and even stuff that we cannot access at all. A lot has changed since that request was made in 2009 and declined in 2013, and it's time to revisit this issue.