I’m playing with the terrain generator, but it somehow only applies terrain to a small part of the mesh instead of the whole mesh (if you have a plane with 9 parts in a 3x3 grid, it only modifies in the center plane instead of the whole mesh, even after joining and subdividing everything together).
Now I have some walls and want to make them look nice… help? 
hi,
for a displaced plane, right to the edges, in the height panel, you need to set edge falloff to no edge fall off.
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I noticed this as well. It happened to me when put the terrain in object mode and scaled all the vertices up a bit. Then all revisions to the terrain were still happening at the previous scale, in the center of this mesh. I solved this by simply scaling the object, not the sub mesh. Now I get edge-to-edge terrain (just like pizza toppings).