Function draw_block_boundary 

pub fn draw_block_boundary(
    bbox: Rect,
    fill: Role,
    stroke: impl Into<RoleStroke>,
    structure: Structure,
    painter: &mut Style<'_, impl Renderer>,
)

A block’s stroked boundary: its outline, plus — for a block that opens a scope — a second line inset within it, the classical contains-a-sheet notation. Every mode that draws a block body draws it through here, so the inset rides whatever stroke and fill that mode chose.