Function scroll_offset_at 

pub fn scroll_offset_at(
    top: f64,
    size: f64,
    total: f64,
    viewport: f64,
    scroll: f64,
    align: ScrollAlign,
    inset: f64,
) -> f64

scroll_offset_for_index for an item at top with extent size in content of length total (O(1) over cached offsets).