The solution is to change the padding color. A quick way to change the padding color is to select "View->Padding Color" and then make your selection from the pop out menu. You can even select a custom color of your own making. In the example at right, I've made the padding color a light blue. Setting the padding color to something distinctive helps to define the boundaries of the image you're editing and lets you see the exact size of the canvas you're working on.