src/java.desktop/share/classes/com/sun/java/swing/plaf/gtk/Metacity.java

@@ -328,11 +328,11 @@
         }
     }
 
     JInternalFrame findInternalFrame(Component comp) {
         if (comp.getParent() instanceof BasicInternalFrameTitlePane) {
-            comp = comp.getParent();
+            comp = ((BasicInternalFrameTitlePane)comp.getParent()).getInternalFrame();
         }
         if (comp instanceof JInternalFrame) {
             return  (JInternalFrame)comp;
         } else if (comp instanceof JInternalFrame.JDesktopIcon) {
             return ((JInternalFrame.JDesktopIcon)comp).getInternalFrame();