diff --git a/collects/mred/private/wx/cocoa/canvas.rkt b/collects/mred/private/wx/cocoa/canvas.rkt
index 03fa4dac..b691eb25 100644
--- a/collects/mred/private/wx/cocoa/canvas.rkt
+++ b/collects/mred/private/wx/cocoa/canvas.rkt
@@ -717,16 +717,7 @@
                 (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 ([on (fix-bitmap-size on w h on-x on-y)])
          (let ([img (bitmap->image on)])
            (atomically
             (set! blits (cons (list x y w h img) blits))
diff --git a/collects/mred/private/wx/common/canvas-mixin.rkt b/collects/mred/private/wx/common/canvas-mixin.rkt
index 97309496..cd1a3280 100644
--- a/collects/mred/private/wx/common/canvas-mixin.rkt
+++ b/collects/mred/private/wx/common/canvas-mixin.rkt
@@ -1,9 +1,11 @@
 #lang racket/base
 (require racket/class
+         racket/draw
          "backing-dc.rkt")
 
 (provide canvas-autoscroll-mixin
-         canvas-mixin)
+         canvas-mixin
+         fix-bitmap-size)
 
 ;; Implements canvas autoscroll, applied *before* platform-specific canvas
 ;; methods:
@@ -160,3 +162,16 @@
       (when (or paint-queued
                 (not (send (get-dc) can-backing-flush?)))
         (do-on-paint #f #f)))))
+
+;; useful for fixing the size of a collecting blit:
+(define (fix-bitmap-size on w h 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))))
diff --git a/collects/mred/private/wx/gtk/canvas.rkt b/collects/mred/private/wx/gtk/canvas.rkt
index 6fadee01..8393b6de 100644
--- a/collects/mred/private/wx/gtk/canvas.rkt
+++ b/collects/mred/private/wx/gtk/canvas.rkt
@@ -560,23 +560,12 @@
               (cons win r))))))
      
      (define/public (register-collecting-blit x y w h on off on-x on-y off-x off-y)
-       (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)))))))
+       (let ([on (fix-bitmap-size on w h on-x on-y)]
+             [off (fix-bitmap-size off w h 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