From Craig Bosma,"Please find attached a fix for shared contexts on OS X, with the Cocoa backend. The NSOpenGLContext* _context member of GraphicsHandleCocoa was never set on successfully creation, so even if the dynamic_cast to GraphicsHandleCocoa succeeds, we get a bogus (NULL) pointer on retrieving the actual NSOpenGLContext* (line 1013).

This should fix the problem, as well as cause GraphicsWindowCocoa to report the correct name "GraphicsWindowCarbon" -> "GraphicsWindowCocoa".
"

src/osgViewer/GraphicsWindowCocoa.mm

@@ -1020,6 +1020,10 @@ bool GraphicsWindowCocoa::realizeImplementation()
         OSG_WARN << "GraphicsWindowCocoa::realizeImplementation :: could not create context" << std::endl;
         return false;
     }
+    
+    // set graphics handle for shared usage
+    setNSOpenGLContext(_context);
+    
     GraphicsWindowCocoaGLView* theView = [[ GraphicsWindowCocoaGLView alloc ] initWithFrame:[ _window frame ] ];
     [theView setAutoresizingMask:  (NSViewWidthSizable | NSViewHeightSizable) ];
     [theView setGraphicsWindowCocoa: this];