racket/gui: change placement of children in panel% and pane%

The `panel%` and `pane%` classes were not originally intended for
direct instantiation; instead, `horizontal-pane[l]%` and
`vertical-pane[l]%` provide basic placements that make sense for
multiple children. Adjusting `pane[l]%` to just overlay all children
seems both sensible and useful (if only one child is shown at a time,
for example).

As suggested by David Nelson.

pkgs/gui-pkgs/gui-doc/scribblings/gui/pane-class.scrbl

@@ -8,12 +8,17 @@ A pane is a both a container and a containee area. It serves only
  cannot be hidden or disabled like a @racket[panel%] object.
 
 A @racket[pane%] object has a degenerate placement strategy for
- managing its children; it places them all in the upper left corner
+ managing its children: it places each child as if it was the only
- and does not stretch any of them. The @racket[horizontal-pane%] and
+ child of the panel.  The @racket[horizontal-pane%] and
- @racket[vertical-pane%] classes provide useful geometry management.
+ @racket[vertical-pane%] classes provide useful geometry management
+ for multiple children.
 
 See also @racket[grow-box-spacer-pane%].
 
+@history[#:changed "1.3" @elem{Changed the placement strategy to
+                               stretch and align children, instead of
+                               placing all children at the top-left
+                               corner.}]
 
 @defconstructor[([parent (or/c (is-a?/c frame%) (is-a?/c dialog%) 
                                (is-a?/c panel%) (is-a?/c pane%))]

pkgs/gui-pkgs/gui-doc/scribblings/gui/panel-class.scrbl

@@ -11,11 +11,15 @@ A panel is a both a container and a containee window. It serves mainly
  object can be hidden or disabled.
 
 A @racket[panel%] object has a degenerate placement strategy for
- managing its children; it places them all in the upper left corner
+ managing its children: it places each child as if it was the only
- and does not stretch any of them.  The @racket[horizontal-panel%]
+ child of the panel.  The @racket[horizontal-panel%] and
- and @racket[vertical-panel%] classes provide useful geometry
+ @racket[vertical-panel%] classes provide useful geometry management
- management.
+ for multiple children.
 
+@history[#:changed "1.3" @elem{Changed the placement strategy to
+                               stretch and align children, instead of
+                               placing all children at the top-left
+                               corner.}]
 
 @defconstructor[([parent (or/c (is-a?/c frame%) (is-a?/c dialog%)
                                (is-a?/c panel%) (is-a?/c pane%))]

pkgs/gui-pkgs/gui-lib/info.rkt

@@ -29,4 +29,4 @@
 
 (define pkg-authors '(mflatt robby))
 
-(define version "1.2")
+(define version "1.3")

pkgs/gui-pkgs/gui-lib/mred/private/wxpanel.rkt

@@ -389,15 +389,37 @@
        [do-place-children
         (lambda (children-info width height)
           (check-place-children children-info width height)
+          (define m (border))
+          (define w (- width (* 2 m)))
+          (define h (- height (* 2 m)))
           (let loop ([children-info children-info])
             (if (null? children-info)
                 null
                 (let ([curr-info (car children-info)])
+                  (define cw (car curr-info))
+                  (define ch (cadr curr-info))
+                  (define w-stretch? (caddr curr-info))
+                  (define h-stretch? (cadddr curr-info))
                   (cons
                    (list
-                    0 0
+                    (if w-stretch?
-                    (car curr-info)     ; child-info-x-min
+                        m
-                    (cadr curr-info))   ; child-info-y-min
+                        (case h-align
+                          [(center) (quotient (- w cw) 2)]
+                          [(right) (- w cw)]
+                          [else 0]))
+                    (if h-stretch?
+                        m
+                        (case v-align
+                          [(center) (quotient (- h ch) 2)]
+                          [(bottom) (- h ch)]
+                          [else 0]))
+                    (if w-stretch?
+                        w
+                        cw)
+                    (if h-stretch?
+                        h
+                        ch))
                    (loop (cdr children-info)))))))])
 
       (define curr-spacing const-default-spacing)