FLTK Keyboard event depends on window size

Question

I want to control an FLTK 1.3.8 application on an RPi via remote control, which sends keyboard events and have created a program that reacts to the codes of the remote. My code:

#include <FL/Fl.H>
#include <FL/Fl_Window.H>
#include <FL/Fl_Box.H>
#include <FL/fl_ask.H>

Fl_Window *win      = NULL;

const int fb_left  = 65361;

class DummyBox : public Fl_Box {
public:
    DummyBox(int X,int Y,int W,int H,const char *L=0) : Fl_Box(X,Y,W,H,L) { }
    int handle(int e) {
        int ret = Fl_Box::handle(e);  // assume the buttons won't handle the keyboard events
        int keyCode = Fl::event_key();

        switch(e) {
            case FL_FOCUS:
            case FL_UNFOCUS:
                ret = 1;                // enables receiving keyboard events
                break;
            case FL_SHORTCUT:           // incase widget that isn't ours has focus
            case FL_KEYDOWN:            // keyboard key pushed
            case FL_KEYUP:              // keyboard key released
            {
                switch(keyCode) {
                    case fb_left:  
                        // system("sudo shutdown"); 
                        fl_alert ("sudo shutdown");
                        break;
                }

                break;
            }
        }
        return(ret);
    }
};


int main(int argc, char *argv[]) {
    Fl_Window *win = new Fl_Window(1000,1000);
    new DummyBox(0,0,0,0);
    win->show();
    return(Fl::run());
}

That works. BUT only, if the FL_Window size is about 1000x1000 or greater! It works not for example 500x500.

This seems very incomprehensible to me.