Maybe this has always been this way and I never noticed, but is the mouse pointer in the guest supposed to change to the black OSX cursor when you left-click in the guest? So if you're in the guest, you've got a white Windows pointer but then left-click drag something and it changes to the black OSX pointer.
Looks like this is a new problem with the latest 3846 build. I can confirm this doesn't happen with 3844.