See tester information, description has outdated information
Preview gets pixelated by activating normal lighting
switching to simple lighting removes the effect
effect is more or less severe depending on the block in use
seen on AMD / Intel and Nvidia 775M from reports.
Problem changed slightly from pixelated to black and whitish texture mashup ?
Without normal mapping
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Still not sure what would cause it. Can't confirm for Nvidia, that report might be incorrect if that person was accidentally using intel HD graphics.
EDIT: Nvidia was a mobile card, 775M with old driver 310