Maybe it is the latest Nvidia driver screwing things up. Or a weird interaction with Avast perhaps, and not necessarily Mail Shield per se.
I found that a Flash update I did today also had the same effect. And starting a VM too sometimes. The one that did also has Avast installed in it though. Both before checking my mail.
What I know for sure is that this behaviour started a couple of days ago, and just before that I both updated Avast and my video drivers. So in all likelihood one of those is the culprit, or they somehow clash.
I’ll update when the next driver is released, and when Avast has another update. To see if that fixes it.