Note: before interacting with the horizontally-scrolling containers, make sure you have some browser history by clicking these links in order.

With overscroll-behavior: auto

Use a two-finger swipe gesture on your laptop's trackpad and notice how at the edges of the scroll container, you're invoking the history navigation.

With overscroll-behavior-x: contain

When containing the scroll on the horizontal axis, this unintended effect is removed.