diff --git a/collects/mred/private/wx/cocoa/frame.rkt b/collects/mred/private/wx/cocoa/frame.rkt
index 37d5768d..81f35d67 100644
--- a/collects/mred/private/wx/cocoa/frame.rkt
+++ b/collects/mred/private/wx/cocoa/frame.rkt
@@ -468,7 +468,7 @@
         (tell cocoa enableCursorRects)))
 
     (define/public (flip-screen y)
-      (let ([f (tell #:type _NSRect (tell cocoa screen) frame)])
+      (let ([f (tell #:type _NSRect (tell NSScreen mainScreen) frame)])
         (- (NSSize-height (NSRect-size f)) y)))
 
     (define/override (flip y h) (flip-screen (+ y h)))
diff --git a/collects/mred/private/wx/cocoa/procs.rkt b/collects/mred/private/wx/cocoa/procs.rkt
index db4551a2..e4ac4108 100644
--- a/collects/mred/private/wx/cocoa/procs.rkt
+++ b/collects/mred/private/wx/cocoa/procs.rkt
@@ -99,7 +99,7 @@
                              [f (if (zero? num)
                                     (tell #:type _NSRect screen visibleFrame)
                                     (tell #:type _NSRect screen frame))])
-                        (set-box! xb (->long (NSPoint-x (NSRect-origin f))))
+                        (set-box! xb ((if (and all? (zero? num)) + -) (->long (NSPoint-x (NSRect-origin f)))))
                         (unless (zero? num)
                           (let* ([screen0 (tell screens objectAtIndex: #:type _NSUInteger 0)]
                                  [f0 (tell #:type _NSRect screen0 frame)])