Originally Posted by Logic Ali
The fact that your demo shows it as undefined should have hinted that it might be worth Googling the issue. Only IE9+ supports it.
Unfortunately the page was tested in IE8, 9 and 10 and each of them called an undefined on the linked page.
But.. I ended up adding getBoundingClientRect for whichever browser and device supports that. Works like a charm and no conflicting with vBulletin so we're all good now.
Thanks for the help though, Logic Ali and vwphillips.
PS: I edited the main post but it disappeared (that new 'Moderated Threads and Posts' system is waaaay heavy). When it's approved again, could anyone mark this as resolved for me? Thanks!