Basically a rounded chunk taken out of the left side of my monitor so i can preview what my work will look like.
An optional virtual thumb moving about the screen would enhance the simulation provided we can adjust the skin tone/gender.
The iOS simulator is basically this. It's included with XCode.
Thinking about this also reminds me of the time I found a game controller I physically could not use, because the face buttons were nestled into a little recess. I could only press them about 10% of the way down before my thumbnail clicked against the housing.
Frankly, a screen that isn't rectangluar is awfully weird to me. I wonder if we'll a) all get over it or b) make the sensors smaller to get the rectangular screen back or c) move on to virtualized screens projected from small wearables and forget this whole thing even happened.