If I drag a browser window to the right, making it cover the right-hand half of my monitor, it serves no useful purpose for "centered" content to migrate to the left edge of the window (the physical midpoint of the screen).
Centered means centered within the virtual viewport, not the physical screen.
There are kiosks used for public browsing, where the user has access to full browser app window and can change elements.
There are also ways to get around kiosk mode restrictions, or at least there were for Windows in kiosk mode back in the day. We used it to get browsing access or to just mess with the kiosk. The fun!
And that's probably by far the most value those kiosks ever provided to society - the chance of overriding whatever malicious, socially harmful marketing bullshit purpose they overtly served, and do something fun or useful with them instead.
Alas, opportunities like this are disappearing, as security maximalists are running the show in our industry these days.
It's a cute trick, please don't use it, this is one of many reasons why. If I have a sidebar open, it's because that space is not yours to care about.
Drawing UI is hard. Firefox likes to draw the websites underneat the scrollbar. Google likes to draw system nabigation buttons over the window with all kind of funny interractions. That's what happenes when you train people "on the job".
This was a choice that they actively made. It's not hard by default, they just chose to do something stupid because they think it looks pretty.
For anyone not aware of how bad google is getting about this: Did you know that "sign in with Google" panel that appears in the top right corner of websites is actually part of Chrome rather than the website?
This in blasphemy. Never, ever, think about the user. Users are idiots anyway. The apps shall be used for their intended purposes. All windows shall be maximized ( because we say so). /s