Function widget_ak_id 

pub fn widget_ak_id<S>(
    node_map: &HashMap<NodeId, *mut dyn Widget, S>,
    taffy_id: NodeId,
) -> Option<NodeId>
where
    S: BuildHasher,

The AccessKit id of the widget laid out at taffy_id, or None when that taffy node holds no live widget.

The host tracks focus by taffy::NodeId while the exported tree is keyed by AccessKit id, so anything crossing between the two — moving focus to a node, asserting on one in a test — has to translate. Exposed rather than open-coded so a caller cannot drift from widget_node_id.