Black Arrow on iPad Pro Screen (Not GPS)

A permanent black square has suddenly appeard in the upper-right area of my iPad Pro's screen, and there is a pale grew outline of an arrow pointing up in this square. Note that this is not the Location Sevices (GPS) open arrow icon that is much smaller and higher up on the top banner of the screen (along with the Bluetooth and battery icons). The black square doesn't seem to do anything - no response when I tap-and-hold on the square, nor when I give it a single or double tap. "Pinching" the screen shrinks everything displayed on it, except the black square that seems to be permanent. I hope someone might be able to tell me, please, what this box is all about, and how I can get rid of it. Thanks.