A pane is a window-like object that knows how to behave as a component of an application frame. That is, it supports the pane protocol operations for layout.
Panes come in many different varieties. For example, gadget panes include such things as buttons and scroll bars. Stream panes deal specifically with text. Some panes are defined only in terms of their functionality, without regard to their specific appearance or implementation. These panes are called abstract panes. The abstract definition allows various instances of the pane class to take on a platform-dependent look and feel. Panes can also be classified according to their role in pane hierarchies. Panes that can have child panes are called composite panes; those that cannot are called leaf panes. Composite panes that are in charge of spatially organizing their children are called layout panes.
The address book application frame shows a typical pane hierarchy. There are three instances of text panes that have associated scroll bars. For every extended stream pane, or text field, with affiliated scroll bar panes there is an "invisible" parent pane, known as a scroller pane, which does such things as control the layout of the child panes and ensure that its children are given the space they need.
The ability to address space allocation and composition concerns is the primary characteristic that sets panes apart from their superclass, sheets, to be discussed in 1.4.1.3, Sheets. Panes, therefore, understand how much screen space they want and need. For instance, the menu pane in the address book application has a static height, so that if the window is resized, the menu pane will not be scaled vertically. On the other hand, the scroller pane labeled in 1.4.1.2 (the pane controlling the application pane for the address book index and the gadget panes for the two associated scroll bars) can be resized as long as the scroll bars are granted enough screen space to function, that is to say, to display the minimum graphics necessary to implement scrolling.
Common Lisp Interface Manager 2.0 User's Guide - 7 Aug 2017