have container retain shown children

original commit: fdf38124a50108d2310ee600803656f0d791f774

collects/mred/private/wx/gtk/canvas.rkt

@@ -179,7 +179,7 @@
           [gl-config #f])
 
     (inherit get-gtk set-size get-size get-client-size 
-             on-size register-as-child get-top-win
+             on-size get-top-win
              set-auto-size adjust-client-delta)
 
     (define is-combo? (memq 'combo style))
@@ -347,12 +347,9 @@
     (define/public (queue-backing-flush)
       (gtk_widget_queue_draw client-gtk))
     
-    (define/public (reset-child-dcs)
+    (define/override (reset-child-dcs)
       (when (dc . is-a? . dc%)
         (reset-dc)))
-    (define/override (maybe-register-as-child parent on?)
-      (register-as-child parent on?)
-      (when on? (reset-child-dcs)))
 
     (send dc start-backing-retained)
 

collects/mred/private/wx/gtk/frame.rkt

@@ -228,6 +228,15 @@
                "eventspace has been shutdown"))
       (super show on?))
 
+    (define saved-child #f)
+    (define/override (register-child child on?)
+      (unless on? (error 'register-child-in-frame "did not expect #f"))
+      (unless (or (not saved-child) (eq? child saved-child))
+        (error 'register-child-in-frame "expected only one child"))
+      (set! saved-child child))
+    (define/override (register-child-in-parent on?)
+      (void))
+
     (define/override (direct-show on?)
       (super direct-show on?)
       (register-frame-shown this on?))

collects/mred/private/wx/gtk/panel.rkt

@@ -17,7 +17,6 @@
 
 (define (panel-mixin %)
   (class %
-    (inherit register-as-child)
 
     (define lbl-pos 'horizontal)
     (define children null)
@@ -27,7 +26,7 @@
     (define/public (get-label-position) lbl-pos)
     (define/public (set-label-position pos) (set! lbl-pos pos))
 
-    (define/public (reset-child-dcs)
+    (define/override (reset-child-dcs)
       (when (pair? children)
         (for ([child (in-list children)])
           (send child reset-child-dcs))))
@@ -35,10 +34,6 @@
     (define/override (set-size x y w h)
       (super set-size x y w h)
       (reset-child-dcs))
-    
-    (define/override (maybe-register-as-child parent on?)
-      (register-as-child parent on?)
-      (when on? (reset-child-dcs)))
 
     (define/override (register-child child on?)
       (let ([now-on? (and (memq child children) #t)])

collects/mred/private/wx/gtk/window.rkt

@@ -3,6 +3,7 @@
          racket/class
          ffi/unsafe/atomic
          "../../syntax.rkt"
+         "../../lock.rkt"
          "../common/event.rkt"
          "../common/freeze.rkt"
          "../common/queue.rkt"
@@ -370,13 +371,17 @@
 
     (define shown? #f)
     (define/public (direct-show on?)
-      (if on?
-          (gtk_widget_show gtk)
-          (gtk_widget_hide gtk))
-      (set! shown? (and on? #t))
-      (maybe-register-as-child parent on?))
+      (as-entry
+       (lambda ()
+         (if on?
+             (gtk_widget_show gtk)
+             (gtk_widget_hide gtk))
+         (set! shown? (and on? #t))
+         (register-child-in-parent on?)))
+      (when on? (reset-child-dcs)))
     (define/public (show on?)
       (direct-show on?))
+    (define/public (reset-child-dcs) (void))
     (define/public (is-shown?) shown?)
     (define/public (is-shown-to-root?)
       (and shown?
@@ -454,12 +459,11 @@
 
     (define/public (on-size w h) (void))
 
-    (define/public (maybe-register-as-child parent on?)
-      (void))
-    (define/public (register-as-child parent on?)
-      (send parent register-child this on?))
     (define/public (register-child child on?)
       (void))
+    (define/public (register-child-in-parent on?)
+      (when parent
+        (send parent register-child this on?)))
 
     (def/public-unimplemented on-drop-file)
     (def/public-unimplemented get-handle)