Function nth_visible_line_forward 

pub fn nth_visible_line_forward(
    fold_idx: &FoldIndex,
    start_line: u32,
    offset: u32,
    total_lines: u32,
) -> u32

Walk forward from start_line and return the buffer line at offset visible (non-fold-hidden) display rows below it. Closed fold bodies are skipped — only fold heads (start lines) and non-folded lines count as visible rows. When the walk reaches the last addressable line before consuming offset rows, that last line is returned (the caller’s clamp).