Fltk channel receiver misses some messages - rust

Question

I create an fltk window with a button. When clicked, the button spawns a thread with 10 iterations inside. Each iteration sends a message to the channel. With println!() everything works fine, but if I delete it - the channel starts skipping some messages.
[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);
}
}
}
The output (without println!() in the thread):
RECEIVED: 1
RECEIVED: 3
RECEIVED: 5
RECEIVED: 7
RECEIVED: 9
With 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

Answer 1

So, after few days of learning magic, I decided to do "cargo update + build" and then everything started working properly.
I still don't quite understand what was that and how did the previous downloaded dependencies get corrupted, but.. yeah, updating fixed it
Another question, I still dont get why sometimes the channel receives all the 10 messages immediately, and sometimes 0123 [1-2 seconds pause] 456789. But I think I will eventually find it out
Thanks to everyone who took my code and tested it, especially #denys-séguret, #joe-jingyu