the last parameter of XChangeProperty is the number of elements.. and when the element format is 32.. the element is "long" so we have 5 long elements here.

Yes this seems confusing as on mac+linux Long is either 32 or 64bits depending on the architecture, but this is how the X11 protocol is defined. Thus 5 is the correct value for the nelts here.  Not 5 or 10 depending on the architecture.

More info on the confusion https://bugs.freedesktop.org/show_bug.cgi?id=16802

test/testnative.h

@@ -0,0 +1,46 @@
+/*
+  Copyright (C) 1997-2014 Sam Lantinga <slouken@libsdl.org>
+
+  This software is provided 'as-is', without any express or implied
+  warranty.  In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely.
+*/
+
+/* Definitions for platform dependent windowing functions to test SDL
+   integration with native windows
+*/
+
+#include "SDL.h"
+
+/* This header includes all the necessary system headers for native windows */
+#include "SDL_syswm.h"
+
+typedef struct
+{
+    const char *tag;
+    void *(*CreateNativeWindow) (int w, int h);
+    void (*DestroyNativeWindow) (void *window);
+} NativeWindowFactory;
+
+#ifdef SDL_VIDEO_DRIVER_WINDOWS
+#define TEST_NATIVE_WINDOWS
+extern NativeWindowFactory WindowsWindowFactory;
+#endif
+
+#ifdef SDL_VIDEO_DRIVER_X11
+#define TEST_NATIVE_X11
+extern NativeWindowFactory X11WindowFactory;
+#endif
+
+#ifdef SDL_VIDEO_DRIVER_COCOA
+/* Actually, we don't really do this, since it involves adding Objective C
+   support to the build system, which is a little tricky.  You can uncomment
+   it manually though and link testnativecocoa.m into the test application.
+*/
+#define TEST_NATIVE_COCOA
+extern NativeWindowFactory CocoaWindowFactory;
+#endif