By being accidentally quadratic or, more likely, naively.

Seems a bit odd to kick down like that - I'd be more inclined to assume it's accidental rather than naive, given the experience level of the author.

They accidentally didn't notice that scrolling is broken enough to bring a supercomputer to its knees?

...it doesn't bring my laptop to its knees, so no, I would wager accidental.

Sometimes I don't even want to know. I once found a website which messed with the scrolling to make it "smoother" - but it did that only when I scrolled with the keyboard (using a Vim plugin). I have no idea how they managed to do that, but at least it got fixed a couple months later.

Centering the background....