From Stephan Huber, "attached you'll find a small enhancement for GraphicsWindowCocoa, so

osgViewer behaves smarter, when the computer will reboot or shutdown. In
older versions the reboot/shutdown got cancelled by GraphicsWindowCocoa,
now it behaves more system conform.
"

src/osgViewer/GraphicsWindowCocoa.mm

@@ -206,7 +206,7 @@ static NSRect convertToQuartzCoordinates(const NSRect& rect)
 {
     s_quit_requested = true;
     DEBUG_OUT("quit requested ");
-    return NSTerminateCancel;
+    return NSTerminateNow;
 }
 
 - (void)applicationDidFinishLaunching:(NSNotification *)aNotification