A black rectangle about 2cm high by 1cm width has appeared at the top left of the screen, looks like a timer of some sort as when clicked it shows in the centre of the rectangle a very quick countdown from 100 to zero. There's a cross at the bottom of the rectangle. It's a nuisance and does not disappear when Surface is closed down and rebooted. Gets in the way of all programs. How to get rid of this?