MANY THANKS to Fluffy, and to Rolig, and Lindal for being so patient with this bear of very little brain. Thanks to Qie for pointing me to the place to set the external editor. My first couple of attempts to trigger VSCode on MacOS have failed, but at least I know what to fiddle with to try to get that working.
So to sum it all up in SL terms: The edit button is only there as an RLV trap to bufuddle scripting noobs.
It's the U/I equivalent of a deep hole with pointy sticks in the bottom camoflaged by twigs and leaves.