Function diff_layout_width 

pub const fn diff_layout_width(
    available_width: usize,
    line_number_width: usize,
    show_gutter: bool,
) -> usize

Width to pass to semantic unified layout when the renderer hides its visible gutter.

layout_display_lines subtracts the normal gutter before wrapping source text. Giving that width back keeps wrapping aligned with compact rendering without adding another public layout option.