Function render_dragging 

fn render_dragging<C: Canvas>(
    data: &Drawing<'_>,
    painter: &mut Style<'_, C>,
    route_id: RouteId,
    label_id: RouteLabelId,
    preview: FracVal,
)

Draw the drag frame: the stored label distance is stale until the release commits, so suppress it and draw the previewed one in its place.