I tried to make a picture of the FD. It's hard to get the camera in there and to get everything in focus, this is the best I could do:



What you can (hopefully) see here is that the chain is on the middle ring, the shifter is in the highest gear. With limit screw / tension as shown, chain rubs against the inner wall of the cage, but does not yet shift up. At the same time, there's already a sizable gap between the outer wall of the cage and the outer ring. If the cage is moved any further to force shifting, the chain will go over the ring and fall off.