What is the point in posting this message to windows windows?
Sometimes, when you lose focus, the pop-up menu in System Tray does not close when you lose focus. Therefore, when processing messages for a pop-up menu, you need to bring a window to the foreground and send him a WM_NULL message.
WM_NULL is often used in hooks, when you cannot "cancel" a message, you can only change it ,
Here, so that the changed message does not find its destination in the hierarchy of window handlers, it is marked with WM_NULL