more cocoa refresh refinements

original commit: 31e46eb8b3ef9a0a0f4ed3c18580718c9b6520ad

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

@@ -223,6 +223,7 @@
         (set! paint-queued #f)
         (when (or (not b) (is-shown-to-root?))
           (send dc reset-backing-retained) ; start with a clean slate
+          (send dc ensure-ready)
           (let ([bg (get-canvas-background)])
             (when bg 
               (let ([old-bg (send dc get-background)])

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

@@ -562,8 +562,7 @@
         ;; in atomic mode
         (when (unbox req) 
           (set-box! req #f)
-          (tellv cocoa-win enableFlushWindow)
-          (tellv cocoa-win flushWindowIfNeeded))))
+          (tellv cocoa-win enableFlushWindow))))
      req)))
 
 (define (cancel-flush-delay req)
@@ -572,10 +571,6 @@
      (when cocoa-win
        (set-box! req #f)
        (tellv cocoa-win enableFlushWindow)
-       (add-event-boundary-sometimes-callback! 
-        cocoa-win
-        (lambda (v)
-          (tellv cocoa-win flushWindowIfNeeded)))
        (remove-event-boundary-callback! req)))))
 
 (define (make-init-point x y)

collects/mred/private/wx/common/backing-dc.rkt

@@ -90,6 +90,8 @@
 
     (define/public (get-bitmap%) bitmap%)
 
+    (define/public (ensure-ready) (get-cr))
+
     (define/override (get-cr)
       (or retained-cr
           (let ([w (box 0)]

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

@@ -347,6 +347,7 @@
                                    (set! paint-queued? #f)
                                    (set! now-drawing? #t)
                                    (send dc reset-backing-retained) ; clean slate
+                                   (send dc ensure-ready)
                                    (let ([bg (get-canvas-background)])
                                      (when bg 
                                        (let ([old-bg (send dc get-background)])