Funnily I can't reproduce the effect at all. I am using Firefox and with sidebar the content of the page centers on the remaining viewport width, like any other site does. I have also tested with Edge, same result. With Edge's auto collapse (i.e. non sticky vertical tabs), I get the described effect, but again this is like any other site. I was actually expecting quite bad results in Edge because it has some sidebars on the right and I thought this page compensates in the wrong direction then. Don't have Chrome installed to see if that does anything differently.

Note I like the default behavior of browsers to center on the viewport as opposed to the window treating sidebars as overlays over the page. Whenever pages do that with their own gadgets I hate it, because they often don't move their content out of the way and it makes it unreadable unless scrolled out of the range of these gadgets, which often isn't possible at all. And even when it is, it effectively reduces the total usable screen to a fraction.

Whenever I'm bothered enough (happens mostly on mobile), I use a kill sticky bookmarklet to fix that.