Occasionally in ZBrush, the whole canvas doesn’t get re-drawn. This might be a known bug, or a video card thing.
Normally it occurs when loading in a new project (with the right divider open). Closing the divider (and extending the canvas) that areas doesn’t get re-drawn - shown by the red lines in the attached image.
Without having to re-load any geometry, is there a way to refresh the canvas at this point? 
Cheers