Function plan 

pub fn plan(
    mv: ColumnMove,
    current: usize,
    count: usize,
    room: Room,
    width: impl FnMut(usize) -> Option<u16>,
) -> Option<usize>

Where the scrolling columns start after mv, from current among count. Every move that can go somewhere does: a page moves at least one column, even past a column wider than the room. None when a width it needs has not been drawn.