simple-layout is a layout with a single child, and the child is resized to fill the space (where possible).
A simple layout's description can be either a single child, or a list containing just one child. The simple layout then adopts the size constraints of its child, and lays the child out inside itself.
:description (list (make-instance
CAPI User Guide and Reference Manual (Macintosh version) - 25 Feb 2015