
A panel is a both a container and a containee window. It serves mainly
 as a geometry management device, but the 
'border creates a
 container with a border. Unlike a 
pane% object, a 
panel%
 object can be hidden or disabled.
A panel% object has a degenerate placement strategy for
 managing its children: it places each child as if it was the only
 child of the panel.  The horizontal-panel% and
 vertical-panel% classes provide useful geometry management
 for multiple children.
Changed in version 1.3 of package gui-lib: Changed the placement strategy to
stretch and align children, instead of
placing all children at the top-left
corner.
If the 
'border style is specified, the window is created with
 a thin border (in which case the client size of the panel may be
 less than its total size). If 
style includes 
'deleted, then the panel is created as hidden,
and it does not affect its parent’s geometry; the panel can be made active later by calling
parent’s 
add-child method.
If the 'hscroll or 'vscroll style is specified, then
 the panel includes a scrollbar in the corresponding direction, and
 the panel’s own size in the corresponding direction is not
 constrained by the size of its children subareas. The 'auto-hscroll
 and 'auto-vscroll styles imply 'hscroll and
 'vscroll, respectively, but they cause the corresponding scrollbar to
 disappear when no scrolling is needed in the corresponding direction;
 the 'auto-vscroll and 'auto-hscroll modes assume that
 children subareas are placed using the default algorithm for a panel%,
 vertical-panel%, or horizontal-panel%. The 'hide-hscroll
 and 'hide-vscroll styles imply 'auto-hscroll and
 'auto-vscroll, respectively, but the corresponding scroll bar is never
 made visible (while still allowing the panel content to exceed its own size).
For information about the enabled argument, see window<%>. For information about the horiz-margin and vert-margin
    arguments, see subarea<%>. For information about the border, spacing, and alignment
    arguments, see area-container<%>. For information about the
    min-width, min-height, stretchable-width, and
    stretchable-height arguments, see area<%>.
Changed in version 1.25 of package gui-lib: Added 'hide-vscroll and 'hide-hscroll.