Popup fades in as you scroll, opacity associated with scroll distance directly.

Source: https://www.thealgorithmicbridge.com/p/this-rumor-about-gpt-5-changes-everything

Came across this Exit Intent popup designed to catch your attention with a completely off the wall element you are shocked to see appear.

Definitely caught my attention and caused me to stick around and read it.

Source: https://www.if-so.com/