I just looked at it on like four different devices and didn’t have any problem. It works perfectly for me on Windows 10 with Chrome, Mac High Sierra/Chrome and iOS 11 / Safari iPad and iPhone.
If you don’t believe me, I will post a screen cast on imgur if you’d like.
You can see the jumping on webpagetest.org:
https://www.webpagetest.org/video/compare.php?tests=180616_B...
With a slow connection, on the initial load the first jump happens at 6s and another one at 8.5s.
When I click on one of the links, it happens much faster because everything is cached. But I can still see the jump everytime I click a link to a new section in the tutorial.
I don't think the type of browser plays a role here. I get the glitches on Chrome and Firefox. And testing it on browserling.com shows that they also happen on Internet Explorer.