adjust canvas refresh strategy yet again

- there seems to be no need to auto-resume flushes on a canvas,
   which can create flicker if the auto-resume timeout turns out
   to be too short

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

@@ -272,9 +272,11 @@
      ;; are defined by `canvas-mixin' from ../common/canvas-mixin
      (define/public (queue-paint) (void))
      (define/public (request-canvas-flush-delay)
-       (request-flush-delay (get-cocoa-window)))
+       (unless is-gl?
+         (request-flush-delay (get-cocoa-window))))
      (define/public (cancel-canvas-flush-delay req)
-       (cancel-flush-delay req))
+       (unless is-gl?
+         (cancel-flush-delay req)))
      (define/public (queue-canvas-refresh-event thunk)
        (queue-window-refresh-event this  thunk))
 

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

@@ -26,6 +26,7 @@
     (init [(cnvs canvas)])
     (define canvas cnvs)
 
+    (inherit end-delay)
     (super-new)
 
     (define gl #f)
@@ -59,21 +60,18 @@
         (values (unbox xb) (unbox yb))))
 
     (define/override (queue-backing-flush)
-      ;; With Cocoa window-level delay doesn't stop
-      ;; displays; it blocks flushes to the screen.
-      ;; So leave the delay in place, and `end-delay'
-      ;; after displaying to the window (after which
-      ;; we'll be ready to flush the window), which
-      ;; is at then end of `do-backing-flush'.
+      ;; Re-enable expose events so that the queued 
+      ;; backing flush will be handled:
+      (end-delay)
       (send canvas queue-backing-flush))
 
     (define/override (flush)
       (send canvas flush))
 
     (define/override (request-delay)
-      (request-flush-delay (send canvas get-flush-window)))
+      (send canvas request-canvas-flush-delay))
     (define/override (cancel-delay req)
-      (cancel-flush-delay req))))
+      (send canvas cancel-canvas-flush-delay req))))
 
 (define (do-backing-flush canvas dc ctx dx dy)
   (tellv ctx saveGraphicsState)
@@ -99,6 +97,5 @@
                    (cairo_fill cr)
                    (cairo_set_source cr s)
                    (cairo_pattern_destroy s))
-                 (cairo_destroy cr))))
-           (send dc end-delay)))
+                 (cairo_destroy cr))))))
    (tellv ctx restoreGraphicsState)))

collects/mred/private/wx/common/delay.rkt

@@ -6,19 +6,28 @@
  (protect-out do-request-flush-delay
               do-cancel-flush-delay))
 
+;; Auto-cancel schedules a cancel of a request flush
+;; on event boundaries. It makes sense if you don't
+;; trust a program to un-delay important refreshes,
+;; but auto-cancel is currently disabled because
+;; bad refresh-delay effects are confined to the enclosing
+;; window on all platforms.
+(define AUTO-CANCEL-DELAY? #f)
+
 (define (do-request-flush-delay win disable enable)
   (atomically
    (let ([req (box win)])
      (and 
       (disable win)
       (begin
-        (add-event-boundary-sometimes-callback! 
-         req
-         (lambda (v) 
-           ;; in atomic mode
-           (when (unbox req) 
-             (set-box! req #f)
-             (enable win))))
+        (when AUTO-CANCEL-DELAY?
+          (add-event-boundary-sometimes-callback! 
+           req
+           (lambda (v) 
+             ;; in atomic mode
+             (when (unbox req) 
+               (set-box! req #f)
+               (enable win)))))
         req)))))
 
 (define (do-cancel-flush-delay req enable)
@@ -27,4 +36,5 @@
      (when win
        (set-box! req #f)
        (enable win)
-       (remove-event-boundary-callback! req)))))
+       (when AUTO-CANCEL-DELAY?
+         (remove-event-boundary-callback! req))))))