Constant HANDLE_HIT_HALF_PX 

pub const HANDLE_HIT_HALF_PX: f64 = _; // 10f64

The seam’s hit zone is the slot plus one grab zone of each neighbour (RESIZE_HANDLE_PX inside each window), so every window edge, seam or outer, has the same size of target. Half of the 20 px total each side.