This bug was originally reported here:
https://bugzilla.redhat.com/show_bug.cgi?id=1961119

The issue is that in some circumstances when Desktop.getDesktop().open(File) is being called there might not be any gtk_main() running resulting in the platform's associated application to actually handle opening that file to not show until the Java application actually ends running (exits).

This code in awt_Desktop.c is being used:

/*
 * Class: sun_awt_X11_XDesktopPeer
 * Method: gnome_url_show
 * Signature: (Ljava/lang/[B;)Z
 */
JNIEXPORT jboolean JNICALL Java_sun_awt_X11_XDesktopPeer_gnome_1url_1show
  (JNIEnv *env, jobject obj, jbyteArray url_j)
{
    gboolean success = FALSE;
    const gchar* url_c;

    url_c = (char*)(*env)->GetByteArrayElements(env, url_j, NULL);
    if (url_c == NULL) {
        if (!(*env)->ExceptionCheck(env)) {
            JNU_ThrowOutOfMemoryError(env, 0);
        }
        return JNI_FALSE;
    }

    if (gtk_has_been_loaded) {
        gtk->gdk_threads_enter();
        success = gtk->gtk_show_uri(NULL, url_c, GDK_CURRENT_TIME, NULL);
        gtk->gdk_threads_leave();
    }

In particular:

        gtk->gdk_threads_enter();
        success = gtk->gtk_show_uri(NULL, url_c, GDK_CURRENT_TIME, NULL);
        gtk->gdk_threads_leave();

which fails to call gtk->gtk_main(). This breaks on systems with >= gtk3-3.24.30, for example on Fedora 34.

Steps to reproduce:

$ java TestDesktop /path/to/file.pdf

Actual results:
10 second delay, then the app exits and the file is shown.

Expected results:
No delay, app is opened to show the file immediately.

Additional info:

Running the reproducer with:
$ java TestDesktop --dialog /path/to/file.pdf

... shows the PDF right away because a FileDialog is being opened too which has a gtk_main() call associated with it.