Layers can contain form elements, applets, and plugins. For clarity of communication, this section refers to form elements, applets, and plugins as windowed elements. These elements are special in that they float to the top of all other layers, even if their containing layer is obscured.
If a windowed element is moved to the edge of its containing layer, as soon as any border of the element hits a border of the layer, the element disappears, instead of seeming to glide out of view as non-windowed elements would do. For form elements, it is the individual element that disappears on contact with the border of the layer, not the entire form.
Note however, that windowed elements do move and change visibility in accordance with their containing layer.
Communicator introduces windowless plugins, which are plugins that do not pop to the top of the window and can be drawn below other items in the window. Windowless plugins are discussed in the Plugin documentation. (Cross-reference link to come.)