White border on PSD

Anyone know why a small white border would appear on a PSD whilst working on it???

Might be a video card thing; might be a "feature".

It does not reappear when you create Window --> New window 

see screenshot. Image on left (new window)

image on right with small white border around canvas

both are 128 x 128 image filled with blue pixels - nothing else

It is not a set pixel border as it remains the same size when you zoom in/out.