Function handle_scroll_event 

pub fn handle_scroll_event(
    event: &WidgetEvent,
    axes: &ScrollableAxes,
    scroller: &Rc<RefCell<KineticScroller>>,
    options: &ScrollHandlingOptions,
    ctx: &mut EventContext<'_>,
) -> EventResponse

Apply one scroll event to axes, and answer the boundary question.

Handled means an axis absorbed some of the movement. Ignored means it absorbed none, which is the signal that sends the event to the next container outward — along the pan claimant chain for a finger, up the ordinary bubble for a wheel. OverscrollBehavior::Contain turns a declined scroll into Handled; it never contains the gesture’s end-of-stream bookkeeping, which every claimant on a chain must see.

Returns Ignored unchanged for any event that is not a [WidgetEvent::Scroll], so a caller can chain its own arms after it.