Please describe the problem:
White (255, 255, 255) and black (0, 0, 0) pixels don't smoothly change color
when the contrast is lowered.  But when the slider hits -127, they jump to the
correct color.

Steps to reproduce:
1. Create a new white image
2. Select Layer|Colors|Brightness-Contrast...
3. Slide the contrast slider to -126
At this point the image should be a middle gray (about 130,130,130), but it's
still white.
4. Slide the contrast slider to -127
Now the image will jump to the middle gray
5. Undo the operation to return to a white image
6. Select Layer|Colors|Invert to make a black image
7. Select Layer|Colors|Brightness-Contrast...
8. Slide the contrast slider to -126
At this point the image should be a middle gray (about 126,126,126), but it's
still black.
9. Slide the contrast slider to -127
Now the image will jump to the middle gray.

Actual results:
The white and black remain so until the slider reaches -127, when they jump to
the middle gray.

Expected results:
The white should get progressively darker as the slider is moved to the left. 
The black should get progressively lighter as the slider is moved to the left. 
Near white (254, 254, 254) and near black (1, 1, 1) appear to work correctly.

Does this happen every time?
Yes

Other information:
Win32 version.  Intel Pentium M 1.86 MHz processor.