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/emscripten/joystick-pre.js

@@ -0,0 +1,25 @@
+Module['arguments'] = ['0'];
+//Gamepads don't appear until a button is pressed and the joystick/gamepad tests expect one to be connected
+Module['preRun'].push(function()
+{
+    Module['print']("Waiting for gamepad...");
+    Module['addRunDependency']("gamepad");
+    window.addEventListener('gamepadconnected', function()
+    {
+        //OK, got one
+        Module['removeRunDependency']("gamepad");
+    }, false);
+
+    //chrome
+    if(!!navigator.webkitGetGamepads)
+    {
+        var timeout = function()
+        {
+            if(navigator.webkitGetGamepads()[0] !== undefined)
+                Module['removeRunDependency']("gamepad");
+            else
+                setTimeout(timeout, 100);
+        }
+        setTimeout(timeout, 100);
+    }
+});