java - Flickering overlapped button - GWT -

this class implement simple pics display gwt project

public class photosslideshow extends composite {  private list<image> photos; private int curr; private absolutepanel mainpnl; private simplepanel centerpnl; private image n; private image p; private final int width = 800;  public photosslideshow(final list<string> result) {      curr = 0;     photos = new arraylist<image>();     mainpnl = new absolutepanel();     this.initwidget(mainpnl);     this.setstylename("slideshow");     centerpnl = new simplepanel();      string path = gwt.getmodulebaseurl() + "imageupload?src=";     (string foto : result) {         string url = path.concat(foto);         image.prefetch(url);         final image curr = new image(url);         double ratio = getratio(curr);         curr.setpixelsize(width, (int) (ratio * width));          curr.addmouseoverhandler(new mouseoverhandler() {              @override             public void onmouseover(mouseoverevent event) {                 n.setvisible(true);                 p.setvisible(true);             }          });         curr.addmouseouthandler(new mouseouthandler() {              @override             public void onmouseout(mouseoutevent event) {                 n.setvisible(false);                 p.setvisible(false);             }          });         photos.add(curr);      }      n = new image("images/next.png");     n.setpixelsize(40, 50);     n.setvisible(false);     n.addclickhandler(new clickhandler() {          @override         public void onclick(clickevent event) {             shownext();         }      });      p = new image("images/prev.png");     p.setpixelsize(40, 50);     p.setvisible(false);     p.addclickhandler(new clickhandler() {          @override         public void onclick(clickevent event) {             showprev();         }      });     centerpnl.setwidget(photos.get(curr));     mainpnl.add(centerpnl);     mainpnl.add(n);     mainpnl.add(p);     posizionabottoni();  }  private void shownext() {     if (curr == photos.size() - 1) {         curr = 0;         centerpnl.setwidget(photos.get(curr));     } else {         curr++;         centerpnl.setwidget(photos.get(curr));     }     posizionabottoni(); }  private void showprev() {     if (curr == 0) {         curr = photos.size() - 1;         centerpnl.setwidget(photos.get(curr));     } else {         curr--;         centerpnl.setwidget(photos.get(curr));     }     posizionabottoni(); }  private void posizionabottoni() {     mainpnl.setwidgetposition(p, 0,             (int) (photos.get(curr).getheight() / 2 - p.getheight() / 2));     mainpnl.setwidgetposition(n, (int) (width - n.getwidth()),             (int) (photos.get(curr).getheight() / 2 - n.getheight() / 2)); }  private double getratio(image i) {      system.out.println("dimensioni originali: " + i.getwidth() + " x "             + i.getheight());     double res = (double) i.getheight() / i.getwidth();     system.out.println("ratio: " + res);     return res; }  } 

apparently works pretty fine, there 1 minor problem: when hover pointer on image, buttons correctly appear, move mouse on buttons click them, flicker. know may reason strange behaviour? in advance.