Function three_pane_value 

pub fn three_pane_value(
    max_horizontal: usize,
    max_vertical: usize,
    strategies: &AdaptStrategies,
    history: impl DoubleEndedIterator<Item = PaneRole>,
) -> ThreePaneValue

Which panes get a partition: the latest destination first, then earlier ones, then the rest by priority. With one partition and room for two stacked panes, the first pane that reflows goes under its expanded anchor.