Update: looks like scrollTrigger doesn't want to play nice with 'overflow-y: scroll' in the css
I forked my original pen (the first one linked in this post) and commented out that line, now the animation works - but of course no scroll-snapping.
https://codepen.io/block-head/pen/WNzWWbZ
Incidentally, the JS for scrollTrigger works fine without needing to call the library since (as I assumed in the first place) they are pre-loaded on codepen anyway.
Obviously, I need to sort out the html and css since the color block is moving with the animation, but apart from that I think I'll try scroll snapping using gsap as outlined here: https://css-tricks.com/going-meta-gsap-the-quest-for-perfect-infinite-scrolling/#aa-scroll-snapping/ - presuming that if I keep things within gsap framework everything should work well together...
rather more than I'd bargained for in a first try, but here goes...