Function sidebar_rail_width_fitting 

pub fn sidebar_rail_width_fitting<'a>(
    theme: &Theme,
    text_renderer: &mut TextRenderer,
    labels: impl IntoIterator<Item = &'a str>,
) -> f64

How wide a sidebar’s rail of these destination labels is: its stock width, grown so its longest single word fits whole, up to metrics.rail_max_width. A sidebar the person can widen into a list grows its rail to the word before it shortens one (“Subscriptions”, never “Subscript…”); a fixed rail keeps rail_width_fitting’s cap.