Function region 

pub fn region(
    scene: &mut Scene,
    transform: Affine,
    rect: Rect,
    region: Region,
    theme: &Theme,
) -> AlphaColor<Srgb>

Paint region’s ground over rect and return it, for the region’s contents to measure against (Theme::on_ground, Colors::lift_on, edge_on).

A region is told apart from the content beside it by its tone. Where a material gives the two the same ground (high contrast), the region draws the hairline instead, on the edge it shares with content: the trailing edge of a sidebar, the leading edge of an inspector.