Here is the trouble with runAsync: When application closing it stay in memory yet, as red square button 'stop' on the panel active yet.

Hello.kt

package com.gdetotut

import javafx.application.Application
import javafx.scene.control.Button
import javafx.scene.control.Label
import javafx.scene.layout.VBox
import tornadofx.*
import java.lang.Thread.sleep

class MyApp: App(MyView::class)

class MyView: View() {

    override val root = vbox {
        button("press me...") {
            action {
                runAsyncWithProgress {
                    sleep(1000)
                } ui {
                    text = "yes!!!"
                }
            }
        }

        label("waiting") {

        }
    }

}

fun main(args: Array<String>) {
    Application.launch(MyApp::class.java, *args)
}