The attached file creates two n x n arrays of rectangles (one red, one blue) and a stage that contains the sequence that represents the first array in its scene.content. It then uses a Timeline to switch arrays every s seconds. Various problems occur:
a- sometimes the window has just a title bar and no content, eg, its height and width are 0.

-b sometimes, the switch to the other sequence doesn't work.

-c sometimes the process starts using 100% of the CPU and has to be killed with the Window Task Manager. I think this and b always happen at the same time, but I am not sure of that. This can usually be observed with the interval set to 1s and ncells set to 60.

-d when it is running correctly, sometimes moving another window on top of the program's window, or moving the mouse around the program's window causes c and/or b. This can be seen if the interval is set higher, eg, 3 secs which seems to prevent b/c from happening spontaneously.

-e if you change the run() function so that doit() is called to do the swapping instead of using the Timeline, then no swapping occurs - the rectangles start out blue and never turn red.


package test1;

import javafx.stage.Stage;
import javafx.scene.Scene;
import javafx.scene.Group;
import javafx.scene.shape.Rectangle;
import javafx.scene.text.Text;
import javafx.scene.text.Font;
import javafx.scene.paint.Color;
import javafx.animation.Timeline;
import javafx.animation.KeyFrame;

def ncells = 60;
def interval = 1s;
var count = 0;

var rects1 =
for (i in [0..
ncells * ncells - 1]) {
    Rectangle {
        width:10
        height: 10
        translateX: 12 * (i mod ncells) // ((i - ((i / 50) * 50)) as Integer)
        translateY: 12 * ((
        i / ncells) as Integer)
        fill: Color.BLUE
    }
}

var rects2 =
for (i in [0..
ncells * ncells - 1]) {
    Rectangle {
        width:10
        height: 10
        translateX: 12 * (i mod ncells) // (( i - ((i / 50) * 50)) as Integer)
        translateY: 12 * ((
        i / ncells) as Integer)
        fill: Color.RED
    }
}
var ss = Stage {
    title: bind "{count}, interval = {interval}, ncells = {ncells}"
    width: 700
    height: 700
    scene: Scene {
        content: rects1
    }
}


var tt = Timeline {
    repeatCount: Timeline.INDEFINITE
    keyFrames: [
        KeyFrame {
            time: interval
            action: function() {
                count++;
                if (ss.scene.content == rects1) {
                    println("jj to red: {count}");
                    ss.scene.content = rects2;

                } else {
                    println("jj to blue: {count}");
                    ss.scene.content = rects1;
                }
                println("jj: done");
            }
        }
    ]
}
//tt.play();

function doit() {
    while (true) {
        java.lang.Thread.sleep(3000);
        count++;
        if (ss.scene.content == rects1) {
            println("jj to red: {count}");
            ss.scene.content = rects2;
        } else {
            println("jj to blue: {count}");
            ss.scene.content = rects1;
        }
        println("jj: done")
    }
}

function run() {
    //doit();
    tt.play();
}