A DESCRIPTION OF THE PROBLEM :
I have 8 Mediaplayer playing at the same time. When I seek them at the same time > 50% of the time some videos hang. Sometimes (~30%) after a while they start playing again, but in slow-motion. onError is not called and getRate() = 1.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Play 8 Mediaplayer at the same time, hook them up to the same controls and seek a few times.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
Player all seek to the same time.
ACTUAL -
Player hang without error message. Some will continue at some point in a very slow speed.

---------- BEGIN SOURCE ----------
package org.example;

import javafx.application.Platform;
import javafx.geometry.Insets;
import javafx.geometry.Pos;
import javafx.scene.Node;
import javafx.scene.control.Button;
import javafx.scene.control.Slider;
import javafx.scene.layout.BorderPane;
import javafx.scene.layout.HBox;
import javafx.scene.layout.Pane;
import javafx.scene.layout.Priority;
import javafx.scene.media.MediaPlayer;
import javafx.scene.media.MediaView;
import javafx.stage.Window;
import javafx.util.Duration;

import java.util.ArrayList;
import java.util.List;

public class MediaControlMinimal extends BorderPane {

    private final List<MediaPlayer> mpList;
    private Duration duration;
    private boolean shouldPlay;
    private int setupComplete;

    private final Slider timeSlider;
    private final HBox mediaBar;
    private final Button playButton;
    private final MediaPlayer controlMediaPlayer;

    public MediaControlMinimal(List<MediaPlayer> mpList, int videoRows) {

        this.mpList = mpList;
        List<MediaView> mvList = new ArrayList<>(mpList.size());

        Pane mvPane = new Pane() {
        };

        for (MediaPlayer mp : mpList) {
            MediaView mediaView = new MediaView(mp);
            mvList.add(mediaView);
            mvPane.getChildren().add(mediaView);
        }

        mvPane.setStyle("-fx-background-color: black;");
        setCenter(mvPane);

        mediaBar = new HBox(); // 5 als param für spacing = 5, sieh zeile 247
        mediaBar.setAlignment(Pos.CENTER);
        mediaBar.setPadding(new Insets(5, 10, 5, 10));
        BorderPane.setAlignment(mediaBar, Pos.CENTER);
        playButton = new Button();

        playButton.setOnAction(e -> {
            shouldPlay = !shouldPlay;
            if (shouldPlay) {
                playAll();
            } else {
                pauseAll();
            }
        });

        // Add time slider
        timeSlider = new Slider();
        HBox.setHgrow(timeSlider, Priority.ALWAYS);
        timeSlider.setMinWidth(50);
        timeSlider.setMaxWidth(Double.MAX_VALUE);

        timeSlider.setOnMouseReleased(event -> {
            //timeSlider.setValueChanging(true);
            //timeSlider.setValue((event.getX() / timeSlider.getWidth()) * timeSlider.getMax());
            //timeSlider.setValueChanging(false);
            System.out.println("setOnMouseReleased");
            final Duration seekTo = Duration.millis((event.getX() / timeSlider.getWidth()) * timeSlider.getMax());
            for (MediaPlayer mp : mpList) {
                new Thread(() -> {
                mp.seek(seekTo);
                }).start();
            }
        });

        controlMediaPlayer = mpList.get(1);
        controlMediaPlayer.currentTimeProperty().addListener((observable, oldVal, newVal) -> {
            updateValues(controlMediaPlayer);
        });

        for (MediaPlayer mp : mpList) {
            mp.setOnReady(() -> {
                int videosPerRow = mpList.size() / videoRows;
                if (setupComplete == 0) {
                    duration = mp.getMedia().getDuration();
                    timeSlider.setMax(duration.toMillis());
                    updateValues(mp);
                    final Window window = mvPane.getScene().getWindow();
                    final double titleHeight = window.getHeight() - mvPane.getScene().getHeight();
                    double windowHeight = videoRows * mp.getMedia().getHeight() + titleHeight;
                    if (!Main.isTransDesign) {
                        windowHeight += mediaBar.getHeight();
                    }
                    window.setHeight(windowHeight);
                    window.setWidth(videosPerRow * mp.getMedia().getWidth());
                }

                if (setupComplete < mpList.size()) {
                    final Node mpNode = mvPane.getChildren().get(mpList.indexOf(mp));
                    if (mpList.indexOf(mp) != 0 && mpNode.getLayoutX() == 0 && mpNode.getLayoutY() == 0) {
                        //fenster höhe
                        double xRelocate = mp.getMedia().getWidth() * (mpList.indexOf(mp) % videosPerRow);
                        double yRelocate = mp.getMedia().getHeight() * Math.floorDiv(mpList.indexOf(mp), videosPerRow);
                        mpNode.relocate(xRelocate, yRelocate);
                    }
                    ++setupComplete;
                }
            });
            mp.setCycleCount(MediaPlayer.INDEFINITE);
        }

        mediaBar.getChildren().add(playButton);
        mediaBar.getChildren().add(timeSlider);
        setBottom(mediaBar);
    }

    private void playAll() {
        for (MediaPlayer mp : mpList) {
            mp.play();
        }
    }


    private void pauseAll() {
        for (MediaPlayer mp : mpList) {
            mp.pause();
        }
    }


    protected void updateValues(MediaPlayer mp) {
        if (timeSlider != null) {
            Platform.runLater(() -> {
                Duration currentTime = mp.getCurrentTime();
                timeSlider.setDisable(duration.isUnknown());
                if (!timeSlider.isDisabled() && duration.greaterThan(Duration.ZERO) && !timeSlider.isValueChanging()) {
                    timeSlider.setValue(currentTime.toMillis());
                }
            });
        }
    }
}

package org.example;

import javafx.application.Application;
import javafx.scene.Group;
import javafx.scene.Scene;
import javafx.scene.media.Media;
import javafx.scene.media.MediaPlayer;
import javafx.stage.Stage;

import java.util.ArrayList;
import java.util.List;

public class EmbeddedMediaPlayer extends Application {

    @Override
    public void start(Stage primaryStage) {
        Group root = new Group();


        Scene scene = new Scene(root, 1586, 951);

        List<MediaPlayer> mediaPlayerList = new ArrayList<>();
        // create media player
        for(String s : Main.MEDIA_URL){
            MediaPlayer mediaPlayer = new MediaPlayer(new Media(s));
            mediaPlayer.setAutoPlay(false);
            mediaPlayerList.add(mediaPlayer);
        }
     
        MediaControl mediaControl = new MediaControl(mediaPlayerList, Main.VIDEO_ROWS);
        scene.setRoot(mediaControl);
       
        primaryStage.setScene(scene);
        primaryStage.show();
    }

    @Override
    public void stop(){
       System.exit(0);
    }

    /**
     * The main() method is ignored in correctly deployed JavaFX application.
     * main() serves only as fallback in case the application can not be
     * launched through deployment artifacts, e.g., in IDEs with limited FX
     * support. NetBeans ignores main().
     *
     * @param args the command line arguments
     */
    public static void main(String[] args) {
        launch(args);
    }
}

package org.example;

import java.io.File;
import java.io.FileNotFoundException;
import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class Main {

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

---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
Not really a workaround, but it will make the bug reappear less often: start every seek in a new Thread.

FREQUENCY : often