I’ve added GIFs showing the intended behavior. The brief movement doesn’t bother me, opening or closing a sidebar already resizes the webview and causes pages to reflow in the browsers I use. Ending in the wrong state after repeatedly toggling it is a bug. And yeah, if you prefer viewport centering, there’s no reason you’d want to use the extension.
Not trying to argue you out of your preference, but trying to understand it: Why would you want to be able to accidentally hide parts of the page by having a sidebar open, or extended too far? I don't understand why the page should pretend to take up the whole window and ignore that it does truly live in just a viewport. I get why aesthetically it might be pleasing, but isn't that pretty short-lived when faced with serious practical drawbacks?