Function flatten_reading_order 

pub fn flatten_reading_order(root: &dyn Widget) -> Vec<AccessNode>

The accessibility walk ([crate::a11y_tree::walk]) the AccessKit bridge and the retained tree take, flattened. A widget’s synthetic children (a table’s rows, a keyboard’s keys) follow its widget children, each placed where the widget draws it ([crate::a11y::with_synthetic_bounds]) and followed by the widgets it hosts.