A popup hack allowing to use dynamic strings, basically by creating a hidden Choice widget then getting the MenuItem using the `MenuExt::at(0)` method.

popup_hack.rs

use fltk::{prelude::*, *};

fn main() {
    let mut choice = menu::Choice::default();
        choice.add_choice("File/New");
        choice.add_choice(&format!("{}\t", "Edit/Cut"));
        choice.add_choice("File/Open");
        choice.add_choice(&format!("Edit/{}", "Copy"));
        choice.add_choice("Other|Option");

    let app = app::App::default();
    let mut win = window::Window::default()
        .with_size(400, 300)
        .with_label("Right Click!");

    win.end();
    win.show();
    win.handle(move |_, ev| match ev {
        enums::Event::Push => {
            if app::event_mouse_button() == app::MouseButton::Right {
                let coords = app::event_coords();
                let items = choice.at(0).unwrap();
                match items.popup(coords.0, coords.1) {
                    None => println!("No value was chosen!"),
                    Some(val) => println!("{}", val.label().unwrap()),
                }
                true
            } else {
                false
            }
        }
        _ => false,
    });
    app.run().unwrap();
}