diff --git a/BasiliskII/src/MacOSX/utils_macosx.mm b/BasiliskII/src/MacOSX/utils_macosx.mm
index 55dd4a85..b2c66798 100644
--- a/BasiliskII/src/MacOSX/utils_macosx.mm
+++ b/BasiliskII/src/MacOSX/utils_macosx.mm
@@ -74,11 +74,7 @@ void make_window_transparent(SDL_Window * window)
     NSWindow *cocoaWindow = wmInfo.info.cocoa.window;
     NSView *sdlView = cocoaWindow.contentView;
     sdlView.layer.backgroundColor = [NSColor clearColor].CGColor;
-    if (SDL_GetWindowData(window, "maskLayer") == NULL) {
-        CALayer *maskLayer = [CAShapeLayer layer];
-        sdlView.layer.mask = maskLayer;
-        SDL_SetWindowData(window, "maskLayer", maskLayer);
-        
+    if (SDL_GetWindowData(window, "observing") == NULL) {
         NSNotificationCenter *nc = [NSNotificationCenter defaultCenter];
         [nc addObserverForName:NSWindowDidBecomeKeyNotification object:cocoaWindow queue:[NSOperationQueue mainQueue] usingBlock:^(NSNotification * _Nonnull note) {
             NSWindow *window = (NSWindow*)note.object;
@@ -91,6 +87,12 @@ void make_window_transparent(SDL_Window * window)
             [window setLevel:NSNormalWindowLevel];
             [window setIsVisible:YES];
         }];
+        SDL_SetWindowData(window, "observing", nc);
+    }
+    if (SDL_GetWindowData(window, "maskLayer") == NULL) {
+        CALayer *maskLayer = [CAShapeLayer layer];
+        sdlView.layer.mask = maskLayer;
+        SDL_SetWindowData(window, "maskLayer", maskLayer);
     }
     cocoaWindow.backgroundColor = [NSColor clearColor];
     cocoaWindow.hasShadow = NO;