Project reactor backpressure (buffer size?) issue - project-reactor

Question

My goal is to process gui events in parallel, but only when I have processing power plus the last event must always be processed. I have a panel which can be resized. Every resize produces new event. I want to process new width, height of panel on a computation thread pool (Scheduler newParallel = Schedulers.newParallel("Computation", 4);) in ordered fashion. If none of the threads is available I need to drop the oldest gui events and when thread becomes available it should take the latest from the backpressure queue.
I wrote the test app and I have several issues. After gui events stop to being produced there is still considerable time when processing is done which ultimately will manifest as an unwanted animation effect. My guess is that the backpressure queue size=256 kept the old events and is still processing it but it does not match with the result logs. After producing 561 events only 33 events were processed (why not 256?) with ids [0-32, 560]. Is there a way to change the backpressure buffer size (I could not find a way to do it) or maybe there is totally different way I should approach this task?
I attach test code for recreation.
import java.util.Random;
import java.util.concurrent.TimeUnit;
import javafx.application.Application;
import javafx.beans.value.ChangeListener;
import javafx.scene.Scene;
import javafx.scene.layout.StackPane;
import javafx.stage.Stage;
import reactor.core.publisher.Flux;
import reactor.core.publisher.FluxSink.OverflowStrategy;
import reactor.core.publisher.Mono;
import reactor.core.scheduler.Scheduler;
import reactor.core.scheduler.Schedulers;
public class BackpressureApp extends Application {
public static void main(String[] args) {
launch(args);
}
private int id = 0;
#Override
public void start(Stage stage) throws Exception {
Scheduler computation = Schedulers.newParallel("Computation", 4);
Flux<Width> flux = Flux.create(sink -> {
stage.widthProperty().addListener((ChangeListener<Number>) (observable, oldValue, newValue) -> {
Width width = new Width(id++, newValue.doubleValue());
System.out.println("[" + Thread.currentThread().getName() + "] PUBLISH width=" + width);
sink.next(width);
});
}, OverflowStrategy.LATEST);
flux.concatMap(width -> Mono.just(width).subscribeOn(computation).map(this::process))
.publishOn(Schedulers.single())
.subscribe(width -> {
System.out.println("[" + Thread.currentThread().getName() + "] RECEIVED width=" + width);
});
stage.setScene(new Scene(new StackPane()));
stage.show();
}
public Width process(Width width) {
Random random = new Random();
int next = random.nextInt(1000);
try {
TimeUnit.MILLISECONDS.sleep(next);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("[" + Thread.currentThread().getName() + "] PROCESS width=" + width + " sleep=" + next);
return width;
}
}
class Width {
private final int id;
private final double width;
public Width(int id, double width) {
super();
this.id = id;
this.width = width;
}
public int getId() {
return id;
}
public double getWidth() {
return width;
}
#Override
public String toString() {
return "Width[id=" + id + ", width=" + width + "]";
}
}