Sticky on Scroll with Pure CSS

No interactions. No custom code.
The Other Way with Interactions 2.0
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This method will be smoother than using Interactions 2.0/1.0 and leverages native CSS functionality.
NOTHING HERE
The trick is to understand that the bottom layer is set to "relative" and controls the length of the scrolling by making the page longer and longer. The middle layer is set to "absolute" and rides on top of the bottom layer.