Function render_vertical_scrollbar 

pub fn render_vertical_scrollbar(
    area: Rect,
    buf: &mut Buffer,
    content_rows: usize,
    offset: usize,
)

Draws the vertical scrollbar for content_rows of content scrolled to offset, sized against the rows area can actually show.

The track length is the number of reachable scroll positions rather than the row count, so the thumb reaches the bottom exactly when the last row is visible.