src/java.desktop/macosx/classes/sun/lwawt/macosx/CPlatformWindow.java

@@ -1072,10 +1072,17 @@
         if (peer != null) {
             peer.notifyNCMouseDown();
         }
     }
 
+    /* Do toggle full screen upon double click
+     * on non-client area(i.e., title bar)
+     */
+    private void deliverNCDblClick() {
+        execute(this::_toggleFullScreenMode);
+    }
+
     /*
      * Our focus model is synthetic and only non-simple window
      * may become natively focusable window.
      */
     private boolean isNativelyFocusableWindow() {