ADDITIONAL SYSTEM INFORMATION :
LINUX_X86_64
Java 8 or 11 or 13
JavaFX: 11.0.3 or 13.0.x

A DESCRIPTION OF THE PROBLEM :
Dear JavaFX support team,

The exception happens only on Linux, JavaFX 8, 11+ (Java 11 or 13 not relevant). Important in standalone scenario - without embedding inside Eclipse works fine.

Somehow JavaFX thread check fails inside "SWT" environment.

Stacktraces:
java.lang.IllegalStateException: Not on FX application thread; currentThread = main
at javafx.graphics/com.sun.javafx.tk.Toolkit.checkFxUserThread(Toolkit.java:291)
at javafx.graphics/com.sun.javafx.tk.quantum.QuantumToolkit.checkFxUserThread(QuantumToolkit.java:424)
at javafx.web/javafx.scene.web.WebEngine.checkThread(WebEngine.java:1207)
at javafx.web/javafx.scene.web.WebEngine.<init>(WebEngine.java:839)
at javafx.web/javafx.scene.web.WebEngine.<init>(WebEngine.java:826)
at javafx.web/javafx.scene.web.WebView.<init>(WebView.java:260)
at com.sap.components.controls.html.utils.Browser$1.call(Browser.java:87)
at com.sap.components.controls.html.utils.Browser$1.call(Browser.java:83)
at com.sap.components.controls.html.utils.Helpers$1.run(Helpers.java:36)
at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$10(PlatformImpl.java:428)
at java.base/java.security.AccessController.doPrivileged(Native Method)
at javafx.graphics/com.sun.javafx.application.PlatformImpl.lambda$runLater$11(PlatformImpl.java:427)
at javafx.graphics/com.sun.glass.ui.InvokeLaterDispatcher$Future.run(InvokeLaterDispatcher.java:96)
at org.eclipse.swt.internal.gtk.OS._g_main_context_iteration(Native Method)
at org.eclipse.swt.internal.gtk.OS.g_main_context_iteration(OS.java:1600)
at org.eclipse.swt.widgets.Display.readAndDispatch(Display.java:4349)

...

Exception in thread "JavaFX Application Thread" org.eclipse.swt.SWTException: Invalid thread access
at org.eclipse.swt.SWT.error(SWT.java:4711)
at org.eclipse.swt.SWT.error(SWT.java:4626)
at org.eclipse.swt.SWT.error(SWT.java:4597)
at org.eclipse.swt.widgets.Widget.error(Widget.java:532)
at org.eclipse.swt.widgets.Widget.checkWidget(Widget.java:470)
at org.eclipse.swt.widgets.Control.isVisible(Control.java:4462)
at org.eclipse.swt.widgets.ProgressBar.timerProc(ProgressBar.java:307)
at org.eclipse.swt.widgets.Display.windowTimerProc(Display.java:5811)
at javafx.graphics/com.sun.glass.ui.gtk.GtkApplication._runLoop(Native Method)
at javafx.graphics/com.sun.glass.ui.gtk.GtkApplication.lambda$runLoop$11(GtkApplication.java:277)
at java.base/java.lang.Thread.run(Thread.java:834)


Code related to "com.sap.components.controls.html.utils.Helpers":
package com.sap.components.controls.html.utils;

import java.util.concurrent.Callable;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicReference;
import javafx.application.Platform;


/**
 * Title: <TBD>
 * Description: <TBD>
 * Copyright: Copyright (c) 2004-2014
 * Company: SAP AG
 *
 * @author Mykola Gorbarov (d043357)
 * @version 1.0 (In Support since 2014)
 */

public class Helpers {

    /**
     * Synchronously execute task on FX user thread and return result.
     *
     * @param task to execute on FX user thread
     * @return result of the task
     */
    public static <T> T doTask(final Callable<T> task) {
        final CountDownLatch barrier = new CountDownLatch(1);
        final AtomicReference<T> resultRef = new AtomicReference<T>();

        Runnable r = new Runnable() {

            public void run() {
                try {
                    resultRef.set(task.call());
                } catch (Throwable t) {
                    t.printStackTrace();
                } finally {
                    barrier.countDown();
                }
            }
        };

        if (Platform.isFxApplicationThread()) {
            r.run();
        } else {
            Platform.runLater(r);
        }

        while (barrier.getCount() > 0) {
            try {
                barrier.await();
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }

        return resultRef.get();
    }


REGRESSION : Last worked in version 11.0.5

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Bit complex, need Eclipse, GUI with JavaFX - you can embed easy swing applicaiton with javafx html control via jfxpanel.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
No exception

CUSTOMER SUBMITTED WORKAROUND :
So far, no workaround

FREQUENCY : always