JDK-8220272: Found the problem + possible fix

[Thiago Milczarek Sayao]

* THE PROBLEM *

In GlassApplication.cpp, the GDK_FOCUS_CHANGE event gets called from the last window to the first on my setup (I suspect "out-of-order" on some setups and even occasionally in the expected order). So the code to keep the latest enabled window fails (refering to WindowStage.java activeWindows) - that's why the "First Window" (on the code to reproduce the bug report) shows on top - because openjfx thinks it was the latest window. In WindowStage.java:

    final void handleFocusDisabled() {
        if (activeWindows.isEmpty()) {
            return;
        }
        WindowStage window = activeWindows.get(activeWindows.size() - 1); //<-- HERE!!
        window.setIconified(false);
        window.requestToFront();
        window.requestFocus();
    }

When there is a WINDOW_MODAL this code gets called because the modal window disable it's parents (so they do not get events - it's correct). So openjfx brings up what it thinks it's the latest window - but since the out of order GDK_FOCUS_CHANGE - it's the wrong window (activeWindows list has not the correct order).

The Gtk docs states GDK_FOCUS_CHANGE is true "if the window has gained the keyboard focus" which may not be directly correlated to openjfx's WindowEvent.FOCUS_GAINED.

* THE FIX *

To fix this, WindowEvent.FOCUS_GAINED must be called in order of which window is shown (because the window is inserted on activeWindows at this event).

So I have modified glass_window.cpp to immediately call WindowEvent.FOCUS_GAINED when showing the window:

void WindowContextBase::set_visible(bool visible) {
    if (visible) {
        gtk_widget_show_all(gtk_widget);

        //JDK-8220272 - fire event first because GDK_FOCUS_CHANGE is not always in order
        if(jwindow && isEnabled()) {
            mainEnv->CallVoidMethod(jwindow, jWindowNotifyFocus, com_sun_glass_events_WindowEvent_FOCUS_GAINED);
        }
    } else {
        gtk_widget_hide(gtk_widget);
        if (jview && is_mouse_entered) {
            is_mouse_entered = false;
            mainEnv->CallVoidMethod(jview, jViewNotifyMouse,
                    com_sun_glass_events_MouseEvent_EXIT,
                    com_sun_glass_events_MouseEvent_BUTTON_NONE,
                    0, 0,
                    0, 0,
                    0,
                    JNI_FALSE,
                    JNI_FALSE);
            CHECK_JNI_EXCEPTION(mainEnv)
        }
    }
}

Did some tests and it seems to fix the problem.

I do not have permission to make a PR, so posting the fix here. Will happily do the PR if required.

Thanks.