Function pane_transitions 

pub fn pane_transitions(
    motions: &[PaneMotion; 3],
    order: &[PaneRole; 3],
    origin: &[Option<PaneRect>; 3],
    target: &[Option<PaneRect>; 3],
    scaffold_width: f32,
) -> [Option<PaneTransition>; 3]

The transition of each role, in PaneRole::ALL order. origin holds the bounds before the change and target those after it.