Hi, thanks for the reply. Not quite the solution I was hoping for, but maybe I can change my approach.
Maybe if I explain further what I was trying to do. In simple terms, all cells in the workbook have been resized to small squares and the VBA is controlling a 'ball' (aka a small cell) that bounces around the screen. What I needed to be able to test for was when the 'ball' goes off the screen. Yes, I know I could restrict the playing area to a predetermined size, but I wanted to give the user complete flexibility over the size of their playing area.
I suspect that Excel does 'know' this, as if you have 'transition keys' enabled via Options and then you hit the tab key, it moves one visible screen width to the right.