A panel is a both a container and a containee window. It serves mainly as a geometry management device, but the 'border creates a container with a border. Unlike a pane% object, a panel% object can be hidden or disabled.
A panel% object has a degenerate placement strategy for managing its children; it places them all in the upper left corner and does not stretch any of them. The horizontal-panel% and vertical-panel% classes provide useful geometry management.
If the 'border style is specified, the window is created with a thin border (only in this case, the client size of the panel may be less than its total size).