This feature adds a set of primitives to allow elements to anchor to overscroll areas of other elements. It further allows these elements to be revealed with swipe gestures, similar to scrollers.
The web provides a rich set of animation properties and tie-ins to scrolling. What appears to be missing from the set is gesture interaction that allows content to be moved around in response to user swiping on the screen. For example, things like sidebar menu or pull to refresh are awkward to express today without script. Notably, it is possibly to polyfill some of these behaviors with Scroll Driven Animations and Snap Points, but the implementation requires insight and an in-depth understanding of these features behave.
Samples: https://flackr.github.io/web-demos/html/overscrollcontainer
Explainers: https://open-ui.org/components/overscroll-actions.explainer