I can add and manipulate a mask on an object while sculpting. The mask behaves as if it is present, but the mask itself is not shown in the display area. I would have expected the inactive region to be shown as dark or differently colored, but it doesn't display at all. See below.
As you can see on the left, there's no indication that there is a mask on the object, but when sculpted it behaves just like one is there. How can I get the mask to display? What are some reasons it might not be shown?
I can not find any of the masking options out of place.

