I was panning around editing a couple of shapes in a file geodatabase and apparently I clicked too quickly because as I was doing a zoom+pan the map window went black and the whole application dimmed (see attached)
The layers in the map are just a simple polygon feature class in a file geodatabase and a large set of geo-referenced TIFFs.
I was editing two adjacent shapes and checking that their verticies were snapped to each other and I was zooming out while clicking onto the next shape and then the app froze up.