Necroing this thread armed with more information. The bug affects Internet Explorer 11 as well and may occur on virtually any website that uses that fullscreen overlay feature.
It is caused by GPU acceleration and a temporary fix is to disable GPU acceleration in Internet Options, Advanced tab (not sure on Edge).
In case it is relevant, I for sure saw it happen on an AMD R9 390 graphics card. It may have also occurred on an Intel HD 530. I think it happened on an AMD R9 290X as well. I don't think it occurred on an ATI HD 5870.
I've also seen it happen on other websites (especially the login screen on Ubisoft's Uplay site) so it isn't unique to XenForo.