Place in Background Behavior
Place in Background
Setting a master to be placed in the background will send the master to the back and lock the widgets in the master into position they were created in. This is useful for creating layouts where elements such as headers and sidebars are in a fixed position.
You can change the default behavior of a master to be placed in the background when it is added to a wireframe by right-clicking a master in the Masters pane and selecting Behavior->Place in Background.