Function resize_edge 

pub fn resize_edge(
    pos: Point<Pixels>,
    inset: Pixels,
    size: Size<Pixels>,
) -> Option<ResizeEdge>

Which edge a press at pos resizes, or None for a press on the content.

inset is the band’s width, and size the whole window’s — the band runs inside both, so the corners are inset squares and each edge is what is left of that side between them.