Please describe the problem:
Does not always perform its function properly.

Steps to reproduce:
1.  Create a new image.
2.  Fill background with white.
3.  Use a 1 pixel pencil with black as the foreground color to randomly place a dozen or so dots on the background layer.  Try to place the dots so that they are spaced apart significantly.  This will make the problem more evident in the steps below.
4.  Duplicate the layer.
5.  Set the duplicate layer blend mode to Lighten Only.
6.  Use the move tool and the cursor keys to move the Lighten Only layer to the left/right/up/down several pixels.  Moving the layer one pixel in any direction should cause the black dots to disappear.  This appears to work properly when a layer is shifted up or down.  But when a layer is shifted left or right multiple pixels, some of the black dots will disappear and reappear.


Actual results:
See above.

Expected results:
See above.

Does this happen every time?
Yes, for the handful of times that I have experimented with it.

Other information:
I first found evidence of this problem while trying to remove some dust from light areas on an image.  I wanted to duplicate the layer, set the mode to lighten only, and then shift the top layer a couple of pixels to make all of the dust disappear, masking the effect in areas where I didn't want it.  When I tried this, not only did I find that some of the dust specs disappeared and reappeared, but I also had vertical black lines appear in random clusters.

I tried the steps above with a light yellow background and a dark blue pencil with the same results.

I also tried the Darken Only mode with a black background and white dots, and that worked OK.