Why is there a band of black at the top of the display?

[russell-taylor]

For both Direct3D and OpenGL examples, when displaying on the screen.
It appears to have a black zone overlaid and blocking part of the viewport.
It happens when the renderOverfillFactor is 1.0, but not when it is 1.2.
It happens whether or not distortion correction tries to read past the end of the texture.