The demo page doesn't work as described in Firefox when I open my bookmarks or history sidebar. It displays a correction offset, but Firefox valiantly defeats this evil hack and makes sure that the box is still at the center of the viewport as the viewport shrinks. It doesn't work in Chromium either when I open devtools as a sidebar. Correction resets to "move pointer" when I do.

I could appreciate the hack from an engineering perspective if it actually worked, but from a user perspective, this is just another example of overambitious anti-designers going out of their way to replace whatever simple, sane and predictable defaults a browser provides out of the box with something incredibly dumb. Browsing has become a rather paranoid activity of constantly second guessing what bespoke bullshit the website authors might have attached to operations that otherwise have previously established sane behaviors provided by the browser by default.