Incidentally, this isn't the same problem. The handle doesn't stick anywhere, only in the last 3 millimetres of travel or so.

With the lens and faceplate off, it looks like the spring is more or less fully compressed over the angular movement it has when the handle is out those last 3 mm or so as it's retracting, so unless I push it in, the remaining spring tension is insufficient to pull in the handle... but it doesn't seem to be actually binding, it's just friction in the rotation.

Of course the vibration doesn't happen in the sled we just had installed in the other vehicle, for a reason which didn't occur to me until now.

The sled in the van doesn't have the "bumps" in it. The one in the car does. Guess what?