You should see the orange cats going under the border.
As you scroll the element below the orange cats should not move.
There should be exactly six unbroken pictures of the two cats visible in the background at all times.
However, as you scroll the element, a new cat should come into view, with its own associated nested scrollbar.
As you scroll the outer element, the inner cat should move up and down with its border.
As you scroll the inner element, the inner cat should not move.
The orange cats should be on a light grey background, the inner cat should have a yellow background.
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