Function reroute_preview_closed 

pub fn reroute_preview_closed(
    waypoints: &[Waypoint],
    geometry: &mut RouteGeometry,
    ends: Endpoints,
    waypoint_delta: GridVec,
    router: &mut ClosedRouter,
) -> Resolution

Non-destructive drag preview: reroute as if every stored waypoint were shifted by waypoint_delta grid cells, WITHOUT mutating the stored waypoints (the group drag is uncommitted). Only geometry’s edges and endpoints are rewritten, and the authored route is untouched. Routes on a frozen ClosedRouter.