The CAPI provides a wide range of built-in panes, but it is still fairly common to need to create panes of your own. In order to do this, you need to specify both the input behavior of the pane (how it reacts to keyboard and mouse events) and its output behavior (how it displays itself). The class output-pane is provided for this purpose.
An output-pane is a fully functional graphics port. This allows it to use all of the graphics ports functionality to create graphics, and it also has a powerful input model which allows it to receive mouse and keyboard input.
output-pane has a subclass pinboard-layout, to which you can add graphic objects, which makes it easier to organize the interaction when it becomes complex. pinboard-layout is probably the more useful class.
CAPI User Guide and Reference Manual (Macintosh version) - 25 Feb 2015