[moonchild]

I think I drew that conclusion from the following: I rendered a full-screen quad using two triangles, and make each fragment display a hash of its threadgroup id. Most threadgroups were arranged in nice, aligned 4x8 rectangles, but near the boundary, they became morphed and distorted so they could stay within the same triangle. That said, it occurs to me now that this could be an opportunistic thing; I am going to try to repeat that experiment, but with many triangles which are all smaller than a single threadgroup.