Function frame 

pub fn frame(
    buf: &mut Buffer,
    area: Rect,
    depth: Depth,
    palette: &Palette,
) -> Rect

Draw a region at a given Depth and return the area left for content.

The fill is painted only when it would be visible against what is already in the buffer. Everything else is the edge, which is why a well still reads as a well on a terminal that cannot colour one.