Function virtual_window 

pub fn virtual_window(
    extents: &[f64],
    gap: f64,
    scroll: f64,
    viewport: f64,
    overscan: usize,
) -> WindowRange

The items to render for a viewport [scroll, scroll + viewport) on the list’s axis (relative to its content start): every item intersecting it (an item ending exactly at scroll counts) plus overscan on each side. virtualWindow.