ExtendSrf does not work when trying to extend the edge next to the red line outward. It does work to move the edge inward (make the surface smaller). The proglem occurs in Rhino 7, 8 and 9 WIP.
ExtendSrf_Problem.3dm (50.5 KB)
If you merge the edges, the extension works. I tried it and it works.
Rather, one thing I never liked about the command is that it extends the surfaces to a certain point, both in and out.