I was able to click the scene with the mouse and then press the page down key to scroll at normal pace in a desktop browser.

But I had to keep clicking every time a new scene loaded, which I consider an unforced error. I'm always so impressed with the level of effort that goes into these types of projects, but so baffled that so many obvious issues always seem to slip by somehow! My guess is that some issue with the scroll hijacking made this difficult to fix, like maybe a loader framework resets the focus and they didn't have an easy way to modify its code or something.