Function hit 

pub fn hit(view: &View, placed: &Placed, at: Position) -> Spot

What the pointer is over, resolved against the frame that was drawn.

Takes the view as well as the frame because two of the answers are about content rather than geometry: which directory a row is, so that a re-sort between the press and the release cannot select a stranger, and how deep it is, so that the expander’s cell is the one it was drawn in.