This seems to have been triggered by attempting to edit a door in a detail view. Rhino(7) froze for a while after I double clicked the door, and when it came back to life after a minute or so, all the walls near the door no longer displayed in wireframe view. Walls from a different session display properly, as the Gif shows. The walls are all of the same style. Saving and reopening Rhino corrected the problem.
