Function active_descendant_path 

pub fn active_descendant_path(access: &AccessNode) -> Option<Vec<u64>>

The key path to the active descendant a widget nominates — the first node in its synthetic subtree marked focused — or None when it nominates none.

build_tree_update uses the same rule to aim the exported focus. This is the other half: the host uses it to aim keyboard activation at the element the user is on, so a canvas widget implements Widget::perform_action once and gets both the assistive-technology and the Enter/arrow paths, instead of having to implement perform_action again for the keyboard.