You can use the initarg
to specify constraints that apply during creation of the element's interface, but not after the interface is displayed.
For example, this creates a window that starts at least 600 pixels high, but can be made shorter by the user, because that initial constraint is transient. However, the permanent constraints on the heights of the two output panes remain in effect:
(list (make-instance 'output-pane
:initial-constraints '(:visible-min-height 600)))
CAPI User Guide (Unix version) - 30 Aug 2011