I am using Chrome 87 on macOS 10.15.7 and I'm not seeing any scroll bars at all on your site, even though I have my Mac preferences set to always show scroll bars.
This is a UI bug due to formatting meant for mobile devices . The scrollbars are tiny in these instances. This isn't by design on desktop and is getting fixed as we speak.
My solutions for the first 54 challenges can be found at https://github.com/mikez302/cryptopals_solutions. I did them in Python, and I think I did a good job of making my solutions easily readable and understandable, although my opinion on that may be biased.
Adblock seems to block that annoying overlay that covers the page. You can also get rid of it by copying and pasting the following line into your browser's web console or debugger: