Function route_to_children 

pub fn route_to_children(
    children: &mut [Box<dyn Widget>],
    nodes: &[NodeId],
    event: &WindowEvent,
    ctx: &mut EventCtx<'_>,
)

Route event through a container’s children, first handler wins. A child latching a drag gets it exclusively, so the drag survives the cursor crossing a peer whose handler would otherwise claim it first; a child holding an open modal owns the input the same way, so nothing standing behind the sheet takes the press meant for it.