intersectionObserver - I would my slide stop to move when an option box reaches one of boundary-side of the container

I'm trying to build an intersection observer allowing my UI to stop a slide transition when the first or last box of included in a container option to reach the boundaries of the container. Hence, the slide is always filled with the box options and it allows a smooth experience from left to right.

A sandbox could be useful, so here my sandbox

Some schemas could also be handy, so here:

But as you will see it doesn't stop smoothly, however I have effectively set my observer to make it stop as I describe in the schema,

Any hint would be great, thanks

from intersectionObserver - I would my slide stop to move when an option box reaches one of boundary-side of the container