While I was configuring a SAG Mill, I accidently grabbed a stack of cobble in the inventory. I ended up moving my mouse over the top of the 3d preview config area and the whole screen turned gray, held item gets rendered relative to top/left corner of the display...

http://i.imgur.com/mjvUo6Z.png

In that picture, my mouse was actually over the top blank/black area of the 3d preview window, the rendered icon for the stack I was holding was not in the same place my mouse coordinates were.

non-persistent, everything works fine when you move your cursor.