A customer raised an issue on avsim that when it was foggy he saw a light blue border around the screen. A quick test showed that he was correct – there was a border around the screen which was the “fog colour”.
Although it is clearly visible on the monitor it is surprisingly hard to capture as a picture in full screen. In the picture below I have therefore used windowed mode and changed the fog colour to bright red so it contrasts with the window boundary.
Happily there is a simple fix for this. If you go into the controller “other dialog” and tick the Axis Color Fix option then as a side effect this will also fix this problem.
Note that you don’t need to enable the axis display in FSX for this to work as a fix.
Steve's FSX Analysis 
Thank you Steve this is great improvement.
That one pixel frame used to really irritate me, I was trying to live with it but it always jumped out when I enter low visibility.
That is a great find I really appreciated.