diff --git a/collects/mred/private/wx/cocoa/canvas.rkt b/collects/mred/private/wx/cocoa/canvas.rkt
index bac0ac13dd..03fa4dacea 100644
--- a/collects/mred/private/wx/cocoa/canvas.rkt
+++ b/collects/mred/private/wx/cocoa/canvas.rkt
@@ -178,7 +178,7 @@
 
 (define canvas%
   (canvas-mixin
-   (class window%
+   (class (canvas-autoscroll-mixin window%)
      (init parent
            x y w h
            style
@@ -195,17 +195,16 @@
               register-as-child
               get-size get-position
               set-focus
-              client-to-screen)
+              client-to-screen
+              is-auto-scroll? get-virtual-width get-virtual-height
+              reset-auto-scroll
+              refresh-for-autoscroll)
 
      (define vscroll-ok? (and (memq 'vscroll style) #t))
      (define vscroll? vscroll-ok?)
      (define hscroll-ok? (and (memq 'hscroll style) #t))
      (define hscroll? hscroll-ok?)
 
-     (define auto-scroll? #f)
-     (define virtual-height #f)
-     (define virtual-width #f)
-
      (define wants-focus? (not (memq 'no-focus style)))
      (define is-combo? (memq 'combo style))
      (define has-control-border? (and (not is-combo?)
@@ -309,8 +308,8 @@
        (when (dc . is-a? . dc%)
          (send dc reset-backing-retained)
          (send dc set-auto-scroll
-               (if auto-scroll? (scroll-pos h-scroller) 0)
-               (if auto-scroll? (scroll-pos v-scroller) 0)))
+               (if (is-auto-scroll?) (scroll-pos h-scroller) 0)
+               (if (is-auto-scroll?) (scroll-pos v-scroller) 0)))
        (when refresh? (refresh)))
 
      (define/override (get-client-size xb yb)
@@ -380,7 +379,7 @@
                   (is-shown-to-root?))
          (atomically (resume-all-reg-blits)))
        (fix-dc)
-       (when auto-scroll?
+       (when (is-auto-scroll?)
          (reset-auto-scroll 0 0))
        (on-size 0 0))
 
@@ -406,69 +405,25 @@
              (get-size w h)
              (do-set-size (unbox x) (unbox y) (unbox w) (unbox h))))))
 
-     (define/public (set-scrollbars h-step v-step
-                                    h-len v-len
-                                    h-page v-page
-                                    h-pos v-pos
-                                    auto?)
-       (cond
-        [auto?
-         (set! auto-scroll? #t)
-         (set! virtual-width (and (positive? h-len) h-len))
-         (set! virtual-height (and (positive? v-len) v-len))
-         (reset-auto-scroll h-pos v-pos)
-         (refresh-for-autoscroll)]
-        [else
-         (let ([a? auto-scroll?])
-           (set! auto-scroll? #f)
-           (when a? (fix-dc))) ; disable scroll offsets
-         (scroll-range h-scroller h-len)
-         (scroll-page h-scroller h-page)
-         (scroll-pos h-scroller h-pos)
-         (when h-scroller
-           (tell (scroller-cocoa h-scroller) setEnabled: #:type _BOOL (and h-step (positive? h-len))))
-         (scroll-range v-scroller v-len)
-         (scroll-page v-scroller v-page)
-         (scroll-pos v-scroller v-pos)
-         (when v-scroller
-           (tell (scroller-cocoa v-scroller) setEnabled: #:type _BOOL (and v-step (positive? v-len))))
-         (set! virtual-width #f)
-         (set! virtual-height #f)]))
+     (define/override (do-set-scrollbars h-step v-step
+                                         h-len v-len
+                                         h-page v-page
+                                         h-pos v-pos)
+       (scroll-range h-scroller h-len)
+       (scroll-page h-scroller h-page)
+       (scroll-pos h-scroller h-pos)
+       (when h-scroller
+         (tell (scroller-cocoa h-scroller) setEnabled: #:type _BOOL (and h-step (positive? h-len))))
+       (scroll-range v-scroller v-len)
+       (scroll-page v-scroller v-page)
+       (scroll-pos v-scroller v-pos)
+       (when v-scroller
+         (tell (scroller-cocoa v-scroller) setEnabled: #:type _BOOL (and v-step (positive? v-len)))))
 
-     (define/private (reset-auto-scroll h-pos v-pos)
-       (let ([xb (box 0)]
-             [yb (box 0)])
-         (get-client-size xb yb)
-         (let ([cw (unbox xb)]
-               [ch (unbox yb)])
-           (let ([h-len (if virtual-width
-                            (max 0 (- virtual-width cw))
-                            0)]
-                 [v-len (if virtual-height
-                            (max 0 (- virtual-height ch))
-                            0)]
-                 [h-page (if virtual-width
-                             cw
-                             0)]
-                 [v-page (if virtual-height
-                             ch
-                             0)])
-             (scroll-range h-scroller h-len)
-             (scroll-page h-scroller h-page)
-             (scroll-pos h-scroller h-pos)
-             (when h-scroller
-               (tell (scroller-cocoa h-scroller) setEnabled: #:type _BOOL (positive? h-len)))
-             (scroll-range v-scroller v-len)
-             (scroll-page v-scroller v-page)
-             (scroll-pos v-scroller v-pos)
-             (when v-scroller
-               (tell (scroller-cocoa v-scroller) setEnabled: #:type _BOOL (positive? v-len)))))))
+     (define/override (reset-dc-for-autoscroll)
+       (fix-dc))
 
-     (define/private (refresh-for-autoscroll)
-       (fix-dc)
-       (refresh))
-
-     (define (update which scroll- v)
+     (define/private (update which scroll- v)
        (if (eq? which 'vertical)
            (scroll- v-scroller v)
            (scroll- h-scroller v)))
@@ -629,7 +584,7 @@
                      'thumb]
                     [else #f])])
               (when kind
-                (if auto-scroll?
+                (if (is-auto-scroll?)
                     (refresh-for-autoscroll)
                     (on-scroll (new scroll-event%
                                     [event-type kind]
@@ -690,22 +645,15 @@
      (define/public (scroll x y)
        (when (x . > . 0) (scroll-pos h-scroller (* x (scroll-range h-scroller))))
        (when (y . > . 0) (scroll-pos v-scroller (* y (scroll-range v-scroller))))
-       (when auto-scroll? (refresh-for-autoscroll)))
+       (when (is-auto-scroll?) (refresh-for-autoscroll)))
 
      (def/public-unimplemented warp-pointer)
 
-     (define/public (view-start xb yb)
-       (if auto-scroll?
-           (begin
-             (set-box! xb (if virtual-width
-                              (scroll-pos h-scroller)
-                              0))
-             (set-box! yb (if virtual-height
-                              (scroll-pos v-scroller)
-                              0)))
-           (begin
-             (set-box! xb 0)
-             (set-box! yb 0))))
+     (define/override (get-virtual-h-pos)
+       (scroll-pos h-scroller))
+
+     (define/override (get-virtual-v-pos)
+       (scroll-pos v-scroller))
 
      (define/public (set-resize-corner on?)
        (void))
@@ -721,11 +669,6 @@
      (define/public (is-flipped?)
        (tell #:type _BOOL (get-cocoa-content) isFlipped))
 
-     (define/public (get-virtual-size xb yb)
-       (get-client-size xb yb)
-       (when virtual-width (set-box! xb virtual-width))
-       (when virtual-height (set-box! yb virtual-height)))
-
      (define blits null)
      (define reg-blits null)
 
diff --git a/collects/mred/private/wx/common/canvas-mixin.rkt b/collects/mred/private/wx/common/canvas-mixin.rkt
index 2316c72775..973094961d 100644
--- a/collects/mred/private/wx/common/canvas-mixin.rkt
+++ b/collects/mred/private/wx/common/canvas-mixin.rkt
@@ -2,11 +2,115 @@
 (require racket/class
          "backing-dc.rkt")
 
-(provide canvas-mixin)
+(provide canvas-autoscroll-mixin
+         canvas-mixin)
 
+;; Implements canvas autoscroll, applied *before* platform-specific canvas
+;; methods:
+(define (canvas-autoscroll-mixin %)
+  (class %
+    (super-new)
+
+    (inherit get-client-size
+             refresh)
+
+    (define auto-scroll? #f)
+    (define virtual-height #f)
+    (define virtual-width #f)
+
+    (define/public (is-auto-scroll?) auto-scroll?)
+    (define/public (get-virtual-height) virtual-height)
+    (define/public (get-virtual-width) virtual-width)
+    
+    (define/public (set-scrollbars h-step v-step
+                                   h-len v-len
+                                   h-page v-page
+                                   h-pos v-pos
+                                   auto?)
+       (cond
+        [auto?
+         (set! auto-scroll? #t)
+         (set! virtual-width (and (positive? h-len) h-len))
+         (set! virtual-height (and (positive? v-len) v-len))
+         (reset-auto-scroll h-pos v-pos)
+         (refresh-for-autoscroll)]
+        [else
+         (let ([a? auto-scroll?])
+           (set! auto-scroll? #f)
+           (set! virtual-width #f)
+           (set! virtual-height #f)
+           (when a? (reset-dc-for-autoscroll))) ; disable scroll offsets
+         (do-set-scrollbars h-step v-step
+                            h-len v-len
+                            h-page v-page
+                            h-pos v-pos)]))
+
+    ;; To be overridden:
+    (define/public (do-set-scrollbars h-step v-step
+                                      h-len v-len
+                                      h-page v-page
+                                      h-pos v-pos)
+      (void))
+
+    (define/public (reset-auto-scroll h-pos v-pos)
+      (let ([xb (box 0)]
+            [yb (box 0)])
+        (get-client-size xb yb)
+        (let ([cw (unbox xb)]
+              [ch (unbox yb)])
+          (let ([h-len (if virtual-width
+                           (max 0 (- virtual-width cw))
+                           0)]
+                [v-len (if virtual-height
+                           (max 0 (- virtual-height ch))
+                           0)]
+                [h-page (if virtual-width
+                            cw
+                            0)]
+                [v-page (if virtual-height
+                            ch
+                            0)])
+            (do-set-scrollbars 1 1
+                               h-len v-len
+                               h-page v-page
+                               h-pos v-pos)))))
+
+    ;; To be overridden:
+    (define/public (reset-dc-for-autoscroll)
+      (void))
+
+    (define/public (refresh-for-autoscroll)
+      (reset-dc-for-autoscroll)
+      (refresh))
+    
+    (define/public (view-start xb yb)
+      (if auto-scroll?
+          (begin
+            (set-box! xb (if virtual-width
+                             (get-virtual-h-pos)
+                             0))
+            (set-box! yb (if virtual-height
+                             (get-virtual-v-pos)
+                             0)))
+          (begin
+            (set-box! xb 0)
+            (set-box! yb 0))))
+    
+    ;; To be overridden:
+    (define/public (get-virtual-h-pos) 0)
+    (define/public (get-virtual-v-pos) 0)
+    
+    (define/public (get-virtual-size xb yb)
+      (get-client-size xb yb)
+      (when virtual-width (set-box! xb virtual-width))
+      (when virtual-height (set-box! yb virtual-height)))))
+
+;; Implements canvas refresh, applied *after* platform-specific canvas
+;; methods:
 (define (canvas-mixin %)
   (class %
     (super-new)
+
     (inherit request-canvas-flush-delay
              cancel-canvas-flush-delay
              queue-canvas-refresh-event
diff --git a/collects/mred/private/wx/gtk/canvas.rkt b/collects/mred/private/wx/gtk/canvas.rkt
index ef389076cb..53c883a849 100644
--- a/collects/mred/private/wx/gtk/canvas.rkt
+++ b/collects/mred/private/wx/gtk/canvas.rkt
@@ -181,7 +181,7 @@
 
 (define canvas%
   (canvas-mixin
-   (class (client-size-mixin window%)
+   (class (canvas-autoscroll-mixin (client-size-mixin window%))
      (init parent
            x y w h
            style
@@ -191,7 +191,10 @@
      (inherit get-gtk set-size get-size get-client-size 
               on-size get-top-win
               set-auto-size 
-              adjust-client-delta infer-client-delta)
+              adjust-client-delta infer-client-delta
+              is-auto-scroll? get-virtual-width get-virtual-height
+              reset-auto-scroll
+              refresh-for-autoscroll)
 
      (define is-combo? (memq 'combo style))
      (define has-border? (or (memq 'border style)
@@ -199,10 +202,6 @@
 
      (define margin (if has-border? 1 0))
 
-     (define auto-scroll? #f)
-     (define virtual-height #f)
-     (define virtual-width #f)
-
      (define-values (client-gtk gtk 
                                 hscroll-adj vscroll-adj hscroll-gtk vscroll-gtk resize-box
                                 combo-button-gtk
@@ -386,10 +385,10 @@
        (send dc reset-backing-retained)
        (refresh)
        (send dc set-auto-scroll
-             (if virtual-width
+             (if (get-virtual-width)
                  (gtk_adjustment_get_value hscroll-adj)
                  0)
-             (if virtual-height
+             (if (get-virtual-height)
                  (gtk_adjustment_get_value vscroll-adj)
                  0)))
 
@@ -438,48 +437,15 @@
                 (gtk_adjustment_configure adj 0 0 1 1 1 1)
                 (gtk_adjustment_configure adj pos 0 (+ len page) 1 page page))))))
 
-     (define/public (set-scrollbars h-step v-step
-                                    h-len v-len
-                                    h-page v-page
-                                    h-pos v-pos
-                                    auto?)
-       (let ([h-page (if (zero? h-len) 0 h-page)]
-             [v-page (if (zero? v-len) 0 v-page)])
-         (cond
-          [auto?
-           (set! auto-scroll? #t)
-           (set! virtual-width (and (positive? h-len) hscroll-gtk h-len))
-           (set! virtual-height (and (positive? v-len) vscroll-gtk v-len))
-           (reset-auto-scroll h-pos v-pos)
-           (refresh-for-autoscroll)]
-          [else
-           (configure-adj hscroll-adj hscroll-gtk h-len h-page h-pos)
-           (configure-adj vscroll-adj vscroll-gtk v-len v-page v-pos)])))
+     (define/override (do-set-scrollbars h-step v-step
+                                         h-len v-len
+                                         h-page v-page
+                                         h-pos v-pos)
+       (configure-adj hscroll-adj hscroll-gtk h-len h-page h-pos)
+       (configure-adj vscroll-adj vscroll-gtk v-len v-page v-pos))
 
-     (define/private (reset-auto-scroll h-pos v-pos)
-       (let ([xb (box 0)]
-             [yb (box 0)])
-         (get-client-size xb yb)
-         (let ([cw (unbox xb)]
-               [ch (unbox yb)])
-           (let ([h-len (if virtual-width
-                            (max 0 (- virtual-width cw))
-                            0)]
-                 [v-len (if virtual-height
-                            (max 0 (- virtual-height ch))
-                            0)]
-                 [h-page (if virtual-width
-                             cw
-                             0)]
-                 [v-page (if virtual-height
-                             ch
-                             0)])
-             (configure-adj hscroll-adj hscroll-gtk h-len h-page h-pos)
-             (configure-adj vscroll-adj vscroll-gtk v-len v-page v-pos)))))
-
-     (define/private (refresh-for-autoscroll)
-       (reset-dc)
-       (refresh))
+     (define/override (reset-dc-for-autoscroll)
+       (reset-dc))
 
      (define/private (dispatch which proc [default (void)])
        (if (eq? which 'vertical)
@@ -559,7 +525,7 @@
      (def/public-unimplemented set-background-to-gray)
 
      (define/public (do-scroll direction)
-       (if auto-scroll?
+       (if (is-auto-scroll?)
            (refresh-for-autoscroll)
            (on-scroll (new scroll-event%
                            [event-type 'thumb]
@@ -572,29 +538,16 @@
         (lambda ()
           (when hscroll-adj (gtk_adjustment_set_value hscroll-adj x))
           (when vscroll-adj (gtk_adjustment_set_value vscroll-adj y))))
-       (when auto-scroll? (refresh-for-autoscroll)))
+       (when (is-auto-scroll?) (refresh-for-autoscroll)))
 
      (def/public-unimplemented warp-pointer)
 
-     (define/public (view-start xb yb)
-       (if auto-scroll?
-           (begin
-             (set-box! xb (if virtual-width
-                              (gtk_adjustment_get_value hscroll-adj)
-                              0))
-             (set-box! yb (if virtual-height
-                              (gtk_adjustment_get_value vscroll-adj)
-                              0)))
-           (begin
-             (set-box! xb 0)
-             (set-box! yb 0))))
+     (define/override (get-virtual-h-pos)
+      (gtk_adjustment_get_value hscroll-adj))
+    (define/override (get-virtual-v-pos)
+      (gtk_adjustment_get_value vscroll-adj))
 
      (define/public (set-resize-corner on?) (void))
-     
-     (define/public (get-virtual-size xb yb)
-       (get-client-size xb yb)
-       (when virtual-width (set-box! xb virtual-width))
-       (when virtual-height (set-box! yb virtual-height)))
 
      (define reg-blits null)