fix gtk collecting blit to revert immediately when gc ends

original commit: bff39a1832548796a06e58d0810827b89a5bc984

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

@@ -595,29 +595,33 @@
 
     (define reg-blits null)
     
-    (define/private (register-one-blit x y w h pixbuf)
+    (define/private (register-one-blit x y w h on-pixbuf off-pixbuf)
       (let* ([cwin (widget-window client-gtk)])
         (atomically
-         (let ([win (create-gc-window cwin x y w h pixbuf)])
+         (let ([win (create-gc-window cwin x y w h)])
            (let ([r (scheme_add_gc_callback
-                     (make-gc-show-desc win pixbuf w h)
-                     (make-gc-hide-desc win))])
+                     (make-gc-show-desc win on-pixbuf w h)
+                     (make-gc-hide-desc win off-pixbuf w h))])
              (cons win r))))))
     
     (define/public (register-collecting-blit x y w h on off on-x on-y off-x off-y)
-      (let ([on (if (and (zero? on-x)
-                         (zero? on-y)
-                         (= (send on get-width) w)
-                         (= (send on get-height) h))
-                    on
-                    (let ([bm (make-object bitmap% w h)])
-                      (let ([dc (make-object bitmap-dc% on)])
-                        (send dc draw-bitmap-section on 0 0 on-x on-y w h)
-                        (send dc set-bitmap #f)
-                        bm)))])
-        (let ([pixbuf (bitmap->pixbuf on)])
-          (atomically
-           (set! reg-blits (cons (register-one-blit x y w h pixbuf) reg-blits))))))
+      (let ([fix-size (lambda (on on-x on-y)
+                        (if (and (zero? on-x)
+                                 (zero? on-y)
+                                 (= (send on get-width) w)
+                                 (= (send on get-height) h))
+                            on
+                            (let ([bm (make-object bitmap% w h)])
+                              (let ([dc (make-object bitmap-dc% on)])
+                                (send dc draw-bitmap-section on 0 0 on-x on-y w h)
+                                (send dc set-bitmap #f)
+                                bm))))])
+        (let ([on (fix-size on on-x on-y)]
+              [off (fix-size off off-x off-y)])
+          (let ([on-pixbuf (bitmap->pixbuf on)]
+                [off-pixbuf (bitmap->pixbuf off)])
+            (atomically
+             (set! reg-blits (cons (register-one-blit x y w h on-pixbuf off-pixbuf) reg-blits)))))))
     
     (define/public (unregister-collecting-blits)
       (atomically

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

@@ -54,7 +54,7 @@
 (define-mz scheme_add_gc_callback (_fun _racket _racket -> _racket))
 (define-mz scheme_remove_gc_callback (_fun _racket -> _void))
 
-(define (create-gc-window cwin x y w h pixbuf)
+(define (create-gc-window cwin x y w h)
   (let ([win (gdk_window_new cwin (make-GdkWindowAttr
                                    ""
                                    0
@@ -68,16 +68,27 @@
                                           GDK_WA_Y))])
     win))
 
+(define (make-draw win pixbuf w h)
+  (vector 'ptr_ptr_ptr_int_int_int_int_int_int_int_int_int->void
+          gdk_draw_pixbuf
+          win #f pixbuf
+          0 0 0 0 w h
+          0 0 0))
+
+(define (make-flush)
+  (vector 'ptr_ptr_ptr->void gdk_display_flush (gdk_display_get_default) #f #f))
+
 (define (make-gc-show-desc win pixbuf w h)
   (vector
    (vector 'ptr_ptr_ptr->void gdk_window_show win #f #f)
-   (vector 'ptr_ptr_ptr_int_int_int_int_int_int_int_int_int->void
-           gdk_draw_pixbuf
-           win #f pixbuf
-           0 0 0 0 w h
-           0 0 0)
-   (vector 'ptr_ptr_ptr->void gdk_display_flush (gdk_display_get_default) #f #f)))
+   (make-draw win pixbuf w h)
+   (make-flush)))
 
-(define (make-gc-hide-desc win)
+(define (make-gc-hide-desc win pixbuf w h)
   (vector
+   ;; draw the ``off'' bitmap so we can flush immediately
+   (make-draw win pixbuf w h)
+   (make-flush)
+   ;; hide the window; it may take a while for the underlying canvas
+   ;; to refresh:
    (vector 'ptr_ptr_ptr->void gdk_window_hide win #f #f)))

collects/scribblings/gui/miscwin-funcs.scrbl

@@ -337,16 +337,15 @@ Registers a ``blit'' to occur when garbage collection starts and
  ends. When garbage collection starts, @racket[on] is drawn at
  location @racket[x] and @racket[y] within @racket[canvas], if
  @racket[canvas] is shown.  When garbage collection ends, the drawing
- is reverted. The @racket[off], @racket[off-x], and @racket[off-y]
- arguments are currently unused, though they were formerly used to
- revert the drawing of @racket[on].
+ is reverted, possibly by drawing the @racket[off] bitmap.
 
 The background behind @racket[on] is unspecified, so @racket[on]
  should be a solid image, and the canvas's scale or scrolling is not
  applied to the drawing. Only the portion of @racket[on] within
  @racket[w] and @racket[h] pixels is used; if @racket[on-x] and
  @racket[on-y] are specified, they specify an offset within the bitmap
- that is used for drawing.
+ that is used for drawing, and @racket[off-x] and @racket[off-y]
+ similarly specify an offset within @racket[off].
 
 The blit is automatically unregistered if @scheme[canvas] becomes
  invisible and inaccessible.  Multiple registrations can be installed