El receptor de canal Fltk no recibe algunos mensajes.

Creo una ventana fltk con un botón. Cuando se hace clic en el botón, se genera un hilo con 10 iteraciones internas. Cada iteración envía un mensaje al canal. Con println!() todo funciona bien, pero si lo elimino, el canal comienza a omitir algunos mensajes.

[dependencies]
fltk = { version="1.2.23", features=["fltk-bundled"] }
fltk-evented = "0.1"

use fltk::{prelude::*, *};
use std::{cell::RefCell, rc::Rc, thread};

fn main() {
    //create channel
    let (mainSender, mainReceiver) = app::channel::<i32>();

    //create application, window, button
    let mainApp = app::App::default();
    let mut win = window::Window::default().with_size(500, 500);
    let mut btn = button::Button::new(20, 20, 100, 40, "START");

    win.end();
    win.show();

    let mainSenderClone = mainSender.clone();

    btn.handle(move |thisBtn, evt| match evt {
        //btn event handler
        enums::Event::Push => {
            //click event

            thread::spawn(move || {
                //create thread

                let mut cnt = 0; //10 iterations
                while (cnt < 10) {
                    mainSenderClone.send(cnt);
                    //println!("sent_from_thread: {}",cnt); - uncommenting this fixes the situation
                    cnt += 1;
                }
            });

            true //event handled
        }

        _ => false, //ignore other events
    });

    //start listening
    while mainApp.wait() {
        if let Some(msg) = mainReceiver.recv() {
            println!("RECEIVED: {}", msg);
        }
    }
}

La salida (sin println!() en el hilo):

RECEIVED: 1
RECEIVED: 3
RECEIVED: 5
RECEIVED: 7
RECEIVED: 9

Con println!():

sent_from_thread: 0
RECEIVED: 0
sent_from_thread: 1
RECEIVED: 1
sent_from_thread: 2
RECEIVED: 2
sent_from_thread: 3
RECEIVED: 3
sent_from_thread: 4
RECEIVED: 4
sent_from_thread: 5
RECEIVED: 5
sent_from_thread: 6
RECEIVED: 6
sent_from_thread: 7
RECEIVED: 7
sent_from_thread: 8
RECEIVED: 8
sent_from_thread: 9
RECEIVED: 9