To see the effect from scroll-snap-stop property, basic scroll snap behaviour must first be established: Scroll-snap-align property is set on child elements to snap on center position, and scroll-snap-type property is set on container to snap vertically.
Now, with scroll-snap-stop property set to always on child elements, scrolling past the next element is prevented when swiping fast with a trackpad or a touch screen.
You need to use a trackpad or a touch screen to see this effect.