Turn off the GPU.
Go to Edit > Preferences > Performance (On a Mac: Lightroom > Preferences > Performance).
Uncheck Use graphics processor.
Two other possibilities:
1. Are you sure the clipping indicators are not on (can't see the full Histogram so they might be). The triangle in the upper right at the top of the Histogram.
2. Make sure you are NOT using an ICC display profile that's been built using a Version 4 (V4) setting. Recalibrate the display, check the settings.
Thanks a lot
This was just what I needed thanks for posting it