A Bootstrap-4 template for Zen Cart versions 1.5.8 through 2.1.0. Forked from https://github.com/zcadditions/ZCA-Bootstrap-Template-for-1.5.6-v2.0.0c. See the demo site, below, for additional information.
Yeah, should only exist on desktop. I looked at walmart.com, amazon.com, pinterest.com (which you can scroll endlessly), and wayfair.com, on my phone and none of them has a back-to-top.
I guess my assumption is that those screen-sizes are associated with mobile/tablet devices which have screen-swipe capability.