overscroll-behavior allows you to override the default scroll behaviour when a browser reaches the boundary of a scrolling area. For example, you are able to disable the 'pull down to refresh' on mobile browsers, or prevent the browser from scrolling the main window when reaching the end of an inner scrollable window (i.e. 'scroll chaining'.
Note that this is a shorthand for overscroll-behavior-x and overscroll-behavior-y.
https://developer.mozilla.org/en-US/docs/Web/CSS/overscroll-behavior