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

premake/projects/testthread.lua

@@ -0,0 +1,28 @@
+-- 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.
+--
+-- Meta-build system using premake created and maintained by
+-- Benjamin Henning <b.henning@digipen.edu>
+
+--[[
+testthread.lua
+
+	This file defines the testthread test project. It depends on the SDL2main
+	and SDL2 projects. It will not build on iOS.
+]]
+
+SDL_project "testthread"
+	SDL_kind "ConsoleApp"
+	SDL_notos "ios"
+	SDL_language "C"
+	SDL_sourcedir "../test"
+	SDL_projectLocation "tests"
+	SDL_projectDependencies { "SDL2main", "SDL2" }
+	SDL_files { "/testthread.*" }