Function demoted_rect 

fn demoted_rect(block: &Block, lowest_slot: u32) -> GridRect

The rect the old root shows as once demoted: the one it already has, made tall enough that its lowest pin slot still fits. Its size is not derived from what it contains — a block as large as the whole drawing is unwieldy — and a degenerate rect (a root written before tops were born with a size) falls back to the default rather than demoting to an invisible sliver.