pub fn focus(view: &View) -> Option<NodeId>Expand description
Which directory the map is of, given where the cursor is.
A row with children maps itself; a claim — which is a leaf — maps its parent, because
a treemap of one rectangle says nothing and what a reader on a leaf wants to know is how
it compares with what is beside it. So drilling in with → re-draws the map one level
down, and landing on a node_modules at the bottom shows the level it lives in.