Function settled_below 

pub fn settled_below(
    frame: Rect,
    occupied: &[Rect],
    gap: f64,
    grid: Grid,
) -> Point

Jumps frame down past whatever is covering it until it clears everything in occupied, and reports where it came to rest.

Each pass goes to the far side of every obstacle it is currently touching, so it retires at least one of them for good — a card taller than the frame’s own row included, which stepping down by that row would never get past. That also bounds the walk by the number of obstacles rather than by the distance.