Function clamp_scroll 

pub fn clamp_scroll(
    selected: usize,
    scroll_offset: usize,
    visible_rows: usize,
    total_rows: usize,
) -> usize

Recompute scroll_offset so selected is inside the visible_rows window. Pure — no Frame access.

• Cursor moved above the window: snap the window to the cursor.
• Cursor moved below the window: scroll just enough to keep it visible (one-line bumps, not page jumps — feels smoother in practice).
• List shrunk: clamp so the offset never points past the end.