R0J0hound is this something that could be fixed with a polyfill? I've looked around, but couldn't find one for it. I assume one could be written, at least for NWjs.
Addition: Also not working on chrome canary or Mozilla nightly.
Addition 2: Bought a new mouse with fully programmable keys, this can get around the problem in one sense by binding mouse 4 to say '4' on the keyboard. However, when binding 'mouse 4' to 'mouse 4', it prevents the button from becoming the browser's back button, but it still does not register. (chrome canary)