In addition to the various layout panes and gadgets, an application usually needs some space to display textual and graphic output as well as to receive application-specific input from the user. For example, a paint program needs a "canvas" pane for displaying the picture and handling "mouse strokes." This can be accomplished in CLIM through the use of extended stream panes .
This section describes the basic CLIM extended stream pane types. Programmers are free to customize pane behavior by defining subclasses of these pane classes. Writing methods to change the repaint or event-handling behavior is a possible starting place.
Common Lisp Interface Manager 2.0 User's Guide - 7 Aug 2017