Falling through parts of terrain. Unity 5

Since Unity 5 doesn't seem to allow previewing the TerrainCollider component I'm having trouble debugging it. I fall through the edges of the terrain, even though it's visually perfectly seamless. I tried to tie them together by putting:

 terraingrid [x, y].GetComponent< TerrainCollider > ().terrainData = terraingrid [x, y].terrainData;

After I've Flushed the seaming changes to the terrain.

I fall off the left, the terrain component is disabled on the right, but the collider is not. There is no preview as you would have with other collider components:

Cube with a disabled mesh renderer.

The terrains in general are seamless.