bindScroll method
Installs the session's scroll listener through PointerSpace, the
one owner of the subscription: wheel/trackpad/second-finger scrolls
move content under a stationary pointer, and the autoscroll ticker's
own jumpTo notifies this same listener, giving one re-resolution
path for every scroll source. detachAll unsubscribes.
Implementation
void bindScroll(VoidCallback listener) {
pointerSpace.bindScroll(listener);
}