Function paint_row_selection_joined 

pub fn paint_row_selection_joined(
    scene: &mut Scene,
    transform: Affine,
    rect: Rect,
    state: SelectionState,
    ground: AlphaColor<Srgb>,
    theme: &Theme,
    join: RowJoin,
)

paint_row_selection for a row inside a run of selected rows: the corners shared with a lifted neighbour are square, so adjacent selected rows join into one lift (a multi-selection in a mail list).