I am new for Ubuntu and gtk+. When I write a sample program to display the menu by gtk, I notice that the menu does not display until I stop the unity-panel-service. It also happen to terminal and File window too. when i type "stop unity-panel-service" the menu buttons of the desktop dis-appear, but the menu of other window show up. Does it is a bug of Ubuntu 16.04 LTS or I do something wrong?