If I switch a ProgressIndicator from indeterminate to 100% complete (done), sometimes it will switch straight away, but most of the time, the GUI will hang without updating for a few seconds before switching to done.

I hope this can be replicated on one of your systems. When I try to capture the behaviour for a screen cast, it works smoothly every time, which is quite strange.

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Sample code: spins for 5 seconds, then switches the progress to 1 (done).

import javafx.animation.PauseTransition;
import javafx.application.Application;
import javafx.scene.Scene;
import javafx.scene.control.ProgressIndicator;
import javafx.stage.Stage;
import javafx.util.Duration;

public class ProgressTrack extends Application {

    @Override public void start(Stage stage) throws Exception {
        ProgressIndicator indicator = new ProgressIndicator();

        stage.setScene(
            new Scene(
                indicator
            )
        );
        stage.show();

        PauseTransition delay = new PauseTransition(Duration.seconds(5));
        delay.setOnFinished(e -> indicator.setProgress(1));
        delay.play();
    }

    public static void main(String[] args) { launch(args); }
}