Function seam_owner 

pub fn seam_owner(seam: &Seam, x: f64) -> EdgeGrab

The seam rule: whose edge a point x on the seam between two windows is.

Every window owns both of its edges, and the seam is split between the two windows, the half nearest each being that window’s edge. The window the person is working in (focused) owns the whole seam: the seam is about two grab zones plus the gap, too small to aim at halves reliably, and “resize the thing I have open” is the person’s own model of it.