Max dirt height defaults to 256 in Customized world (max world height is 255)

The bug

In the second page of the customized world creation menu, the default max height for dirt and gravel patches is 256, but once you change it, you can't put it back there - the slider stops at 255.

How to reproduce

1. Go to the second page of the customized world creation menu.
2. Change the max height for dirt or gravel.
3. Try to put it back to 256. You can't do it manually, but only through the Defaults button.

Most probably the default value is wrong, since the block grid ranges from 0 to 255, and not from 1 to 256 - this means that there's not actually a block layer in Y=256. You can check that by trying to set a block in layer 256.
Example: /setblock ~ 256 ~ stone will return Cannot place blocks outside of the world.