Function expand_flexible_tracks 

fn expand_flexible_tracks<Tree: LayoutPartialTree>(
    tree: &mut Tree,
    axis: AbstractAxis,
    axis_tracks: &mut [GridTrack],
    other_axis_tracks: &[GridTrack],
    items: &mut [GridItem],
    axis_min_size: Option<f32>,
    axis_max_size: Option<f32>,
    axis_available_space_for_expansion: AvailableSpace,
    inner_node_size: Size<Option<f32>>,
    get_track_size_estimate: impl Fn(&GridTrack, Option<f32>, &Tree) -> Option<f32>,
)

11.7. Expand Flexible Tracks This step sizes flexible tracks using the largest value it can assign to an fr without exceeding the available space.