Think about users who don’t maximize the window, for whom centering to the screen, not viewport, would just be broken. It’s right to center to the viewport, and right that you’re able to turn that into centering to the screen by maximizing. The issue, I think, is that you don’t like that the sidebar changes the viewport, and really want a popup menu. I think your browser might duplicate sidebar functionality in configurable toolbar button popups, have you tried that? Failing that, I think the right solution to this problem might be a browser extension.