Function pointer_target 

pub fn pointer_target(
    rects: &HashMap<usize, Rect>,
    from: usize,
    current: Option<usize>,
    at: Point,
    axis: Axis,
) -> Option<usize>

Which row should be the drop target while a pointer drag from row from hovers at at, given per-row rects measured at drag start (so the test runs against the stable, pre-displacement layout). A row is adopted only once the pointer crosses its center in the travel direction, and while the pointer is over the source row or outside every rect, the previous target is kept. Pure, for testability.