@Override
    public int getPrimaryDisplay() {
        long prim = glfwGetPrimaryMonitor();
        Displays monitors = getDisplays();
        for (int i = 0; i < monitors.size(); i++) {
            long monitorI = monitors.get(i).getDisplay();
            if (monitorI == prim) return i;
        }

        LOGGER.log(Level.SEVERE, "Couldn't locate Primary Monitor in the list of Monitors.");
        return -1;
    }
Returns the Primary Monitor position number from the list of monitors
returned by glfwGetPrimaryMonitor().  If primary monitor not found
it will return -1 and report the error.

@return returns the Primary Monitor Position.
jme3-lwjgl3/src/main/java/com/jme3/system/lwjgl/LwjglWindow.java